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Part I. Overview of Spring Framework

The Spring Framework is a lightweight solution and a potential one-stop-shop for
building your enterprise-ready applications. However, Spring is modular, allowing you to
use only those parts that you need, without having to bring in the rest. You can use the
IoC container, with any web framework on top, but you can also use only the
Hibernate integration code or the JDBC abstraction
layer. The Spring Framework supports declarative transaction management, remote access
to your logic through RMI or web services, and various options for persisting your data.
It offers a full-featured MVC framework, and enables you to
integrate AOP transparently into your software.

Spring is designed to be non-intrusive, meaning that your domain logic code generally
has no dependencies on the framework itself. In your integration layer (such as the data
access layer), some dependencies on the data access technology and the Spring libraries
will exist. However, it should be easy to isolate these dependencies from the rest of
your code base.

This document is a reference guide to Spring Framework features. If you have any
requests, comments, or questions on this document, please post them on the
user mailing
list. Questions on the Framework itself should be asked on StackOverflow
(see https://spring.io/questions).

1. Getting Started with Spring

This reference guide provides detailed information about the Spring Framework.
It provides comprehensive documentation for all features, as well as some background
about the underlying concepts (such as "Dependency Injection") that Spring has
embraced.

If you are just getting started with Spring, you may want to begin using the Spring Framework
by creating a Spring Boot based application.
Spring Boot provides a quick (and opinionated) way to create a production-ready Spring based
application. It is based on the Spring Framework, favors convention over configuration, and is
designed to get you up and running as quickly as possible.

2. Introduction to the Spring Framework

The Spring Framework is a Java platform that provides comprehensive infrastructure support
for developing Java applications. Spring handles the infrastructure so you can focus on
your application.

Spring enables you to build applications from "plain old Java objects" (POJOs) and to
apply enterprise services non-invasively to POJOs. This capability applies to the Java
SE programming model and to full and partial Java EE.

Examples of how you, as an application developer, can benefit from the Spring platform:

Make a Java method execute in a database transaction without having to deal with
transaction APIs.

Make a local Java method an HTTP endpoint without having to deal with the Servlet API.

Make a local Java method a message handler without having to deal with the JMS API.

Make a local Java method a management operation without having to deal with the JMX API.

2.1 Dependency Injection and Inversion of Control

A Java application — a loose term that runs the gamut from constrained, embedded
applications to n-tier, server-side enterprise applications — typically consists of
objects that collaborate to form the application proper. Thus the objects in an
application have dependencies on each other.

Although the Java platform provides a wealth of application development functionality,
it lacks the means to organize the basic building blocks into a coherent whole, leaving
that task to architects and developers. Although you can use design patterns such
as Factory, Abstract Factory, Builder, Decorator, and Service Locator
to compose the various classes and object instances that make up an application,
these patterns are simply that: best practices given a name, with a description
of what the pattern does, where to apply it, the problems it addresses, and so forth.
Patterns are formalized best practices that you must implement yourself in your
application.

The Spring Framework Inversion of Control (IoC) component addresses this concern by
providing a formalized means of composing disparate components into a fully working
application ready for use. The Spring Framework codifies formalized design patterns as
first-class objects that you can integrate into your own application(s). Numerous
organizations and institutions use the Spring Framework in this manner to engineer
robust, maintainable applications.

Background

"The question is, what aspect of control are [they] inverting?" Martin Fowler posed
this question about Inversion of Control (IoC)
on his site in 2004. Fowler suggested
renaming the principle to make it more self-explanatory and came up with Dependency
Injection.

2.2 Framework Modules

The Spring Framework consists of features organized into about 20 modules. These modules
are grouped into Core Container, Data Access/Integration, Web, AOP (Aspect Oriented
Programming), Instrumentation, Messaging, and Test, as shown in the following diagram.

Figure 2.1. Overview of the Spring Framework

The following sections list the available modules for each feature along with their
artifact names and the topics they cover. Artifact names correlate to artifact IDs used
in Dependency Management tools.

2.2.1 Core Container

The spring-core and spring-beans modules provide the fundamental
parts of the framework, including the IoC and Dependency Injection features. The
BeanFactory is a sophisticated implementation of the factory pattern. It removes the
need for programmatic singletons and allows you to decouple the configuration and
specification of dependencies from your actual program logic.

The Context (spring-context) module builds on the solid
base provided by the Core and Beans modules: it is a means to
access objects in a framework-style manner that is similar to a JNDI registry. The
Context module inherits its features from the Beans module and adds support for
internationalization (using, for example, resource bundles), event propagation, resource
loading, and the transparent creation of contexts by, for example, a Servlet container.
The Context module also supports Java EE features such as EJB, JMX, and basic remoting.
The ApplicationContext interface is the focal point of the Context module.
spring-context-support provides support for integrating common third-party libraries
into a Spring application context for caching (EhCache, Guava, JCache), mailing
(JavaMail), scheduling (CommonJ, Quartz) and template engines (FreeMarker, JasperReports,
Velocity).

The spring-expression module provides a powerful Expression
Language for querying and manipulating an object graph at runtime. It is an extension
of the unified expression language (unified EL) as specified in the JSP 2.1
specification. The language supports setting and getting property values, property
assignment, method invocation, accessing the content of arrays, collections and indexers,
logical and arithmetic operators, named variables, and retrieval of objects by name from
Spring’s IoC container. It also supports list projection and selection as well as common
list aggregations.

2.2.2 AOP and Instrumentation

The spring-aop module provides an AOP Alliance-compliant
aspect-oriented programming implementation allowing you to define, for example,
method interceptors and pointcuts to cleanly decouple code that implements functionality
that should be separated. Using source-level metadata functionality, you can also
incorporate behavioral information into your code, in a manner similar to that of .NET
attributes.

The separate spring-aspects module provides integration with AspectJ.

The spring-instrument module provides class instrumentation support and classloader
implementations to be used in certain application servers. The spring-instrument-tomcat
module contains Spring’s instrumentation agent for Tomcat.

2.2.3 Messaging

Spring Framework 4 includes a spring-messaging module with key abstractions from the
Spring Integration project such as Message, MessageChannel, MessageHandler, and
others to serve as a foundation for messaging-based applications. The module also
includes a set of annotations for mapping messages to methods, similar to the Spring MVC
annotation based programming model.

The spring-orm module provides integration layers for popular
object-relational mapping APIs, including JPA,
JDO, and Hibernate. Using the spring-orm module you can
use all of these O/R-mapping frameworks in combination with all of the other features
Spring offers, such as the simple declarative transaction management feature mentioned
previously.

The spring-oxm module provides an abstraction layer that supports Object/XML
mapping implementations such as JAXB, Castor, XMLBeans, JiBX and XStream.

The spring-jms module (Java Messaging Service) contains features for producing and
consuming messages. Since Spring Framework 4.1, it provides integration with the
spring-messaging module.

2.2.5 Web

The Web layer consists of the spring-web, spring-webmvc, spring-websocket, and
spring-webmvc-portlet modules.

The spring-web module provides basic web-oriented integration features such as
multipart file upload functionality and the initialization of the IoC container using
Servlet listeners and a web-oriented application context. It also contains an HTTP client
and the web-related parts of Spring’s remoting support.

The spring-webmvc module (also known as the Web-Servlet module) contains Spring’s
model-view-controller (MVC) and REST Web Services implementation
for web applications. Spring’s MVC framework provides a clean separation between domain
model code and web forms and integrates with all of the other features of the Spring
Framework.

The spring-webmvc-portlet module (also known as the Web-Portlet module) provides
the MVC implementation to be used in a Portlet environment and mirrors the functionality
of the Servlet-based spring-webmvc module.

2.2.6 Test

The spring-test module supports the unit testing and
integration testing of Spring components with JUnit or TestNG. It
provides consistent loading of Spring
ApplicationContexts and caching of those
contexts. It also provides mock objects that you can use to test your
code in isolation.

2.3 Usage scenarios

The building blocks described previously make Spring a logical choice in many scenarios,
from embedded applications that run on resource-constrained devices to full-fledged
enterprise applications that use Spring’s transaction management functionality and web
framework integration.

Figure 2.2. Typical full-fledged Spring web application

Spring’s declarative transaction management features make
the web application fully transactional, just as it would be if you used EJB
container-managed transactions. All your custom business logic can be implemented with
simple POJOs and managed by Spring’s IoC container. Additional services include support
for sending email and validation that is independent of the web layer, which lets you
choose where to execute validation rules. Spring’s ORM support is integrated with JPA,
Hibernate and JDO; for example, when using Hibernate, you can continue to use
your existing mapping files and standard Hibernate SessionFactory configuration. Form
controllers seamlessly integrate the web-layer with the domain model, removing the need
for ActionForms or other classes that transform HTTP parameters to values for your
domain model.

Figure 2.3. Spring middle-tier using a third-party web framework

Sometimes circumstances do not allow you to completely switch to a different framework.
The Spring Framework does not force you to use everything within it; it is not an
all-or-nothing solution. Existing front-ends built with Struts, Tapestry, JSF
or other UI frameworks can be integrated with a Spring-based middle-tier, which allows
you to use Spring transaction features. You simply need to wire up your business logic
using an ApplicationContext and use a WebApplicationContext to integrate your web
layer.

Figure 2.4. Remoting usage scenario

When you need to access existing code through web services, you can use Spring’s
Hessian-, Burlap-, Rmi- or JaxRpcProxyFactory classes. Enabling remote access to
existing applications is not difficult.

Figure 2.5. EJBs - Wrapping existing POJOs

The Spring Framework also provides an access and abstraction layer for
Enterprise JavaBeans, enabling you to reuse your existing POJOs and wrap them in
stateless session beans for use in scalable, fail-safe web applications that might need
declarative security.

2.3.1 Dependency Management and Naming Conventions

Dependency management and dependency injection are different things. To get those nice
features of Spring into your application (like dependency injection) you need to
assemble all the libraries needed (jar files) and get them onto your classpath at
runtime, and possibly at compile time. These dependencies are not virtual components
that are injected, but physical resources in a file system (typically). The process of
dependency management involves locating those resources, storing them and adding them to
classpaths. Dependencies can be direct (e.g. my application depends on Spring at
runtime), or indirect (e.g. my application depends on commons-dbcp which depends on
commons-pool). The indirect dependencies are also known as "transitive" and it is
those dependencies that are hardest to identify and manage.

If you are going to use Spring you need to get a copy of the jar libraries that comprise
the pieces of Spring that you need. To make this easier Spring is packaged as a set of
modules that separate the dependencies as much as possible, so for example if you don’t
want to write a web application you don’t need the spring-web modules. To refer to
Spring library modules in this guide we use a shorthand naming convention spring-* or
spring-*.jar, where * represents the short name for the module
(e.g. spring-core, spring-webmvc, spring-jms, etc.). The actual jar file name that
you use is normally the module name concatenated with the version number
(e.g. spring-core-4.3.10.RELEASE.jar).

Each release of the Spring Framework will publish artifacts to the following places:

Maven Central, which is the default repository that Maven queries, and does not
require any special configuration to use. Many of the common libraries that Spring
depends on also are available from Maven Central and a large section of the Spring
community uses Maven for dependency management, so this is convenient for them. The
names of the jars here are in the form spring-*-<version>.jar and the Maven groupId
is org.springframework.

In a public Maven repository hosted specifically for Spring. In addition to the final
GA releases, this repository also hosts development snapshots and milestones. The jar
file names are in the same form as Maven Central, so this is a useful place to get
development versions of Spring to use with other libraries deployed in Maven Central.
This repository also contains a bundle distribution zip file that contains all Spring
jars bundled together for easy download.

So the first thing you need to decide is how to manage your dependencies: we generally
recommend the use of an automated system like Maven, Gradle or Ivy, but you can also do
it manually by downloading all the jars yourself.

WebSocket and SockJS infrastructure, including STOMP messaging support

Spring Dependencies and Depending on Spring

Although Spring provides integration and support for a huge range of enterprise and
other external tools, it intentionally keeps its mandatory dependencies to an absolute
minimum: you shouldn’t have to locate and download (even automatically) a large number
of jar libraries in order to use Spring for simple use cases. For basic dependency
injection there is only one mandatory external dependency, and that is for logging (see
below for a more detailed description of logging options).

Next we outline the basic steps needed to configure an application that depends on
Spring, first with Maven and then with Gradle and finally using Ivy. In all cases, if
anything is unclear, refer to the documentation of your dependency management system, or
look at some sample code - Spring itself uses Gradle to manage dependencies when it is
building, and our samples mostly use Gradle or Maven.

Maven Dependency Management

If you are using Maven for dependency management you don’t even
need to supply the logging dependency explicitly. For example, to create an application
context and use dependency injection to configure an application, your Maven dependencies
will look like this:

That’s it. Note the scope can be declared as runtime if you don’t need to compile
against Spring APIs, which is typically the case for basic dependency injection use
cases.

The example above works with the Maven Central repository. To use the Spring Maven
repository (e.g. for milestones or developer snapshots), you need to specify the
repository location in your Maven configuration. For full releases:

Maven "Bill Of Materials" Dependency

It is possible to accidentally mix different versions of Spring JARs when using Maven.
For example, you may find that a third-party library, or another Spring project,
pulls in a transitive dependency to an older release. If you forget to explicitly declare
a direct dependency yourself, all sorts of unexpected issues can arise.

To overcome such problems Maven supports the concept of a "bill of materials" (BOM)
dependency. You can import the spring-framework-bom in your dependencyManagement
section to ensure that all spring dependencies (both direct and transitive) are at
the same version.

2.3.2 Logging

Logging is a very important dependency for Spring because a) it is the only mandatory
external dependency, b) everyone likes to see some output from the tools they are
using, and c) Spring integrates with lots of other tools all of which have also made
a choice of logging dependency. One of the goals of an application developer is often to
have unified logging configured in a central place for the whole application, including
all external components. This is more difficult than it might have been since there are so
many choices of logging framework.

The mandatory logging dependency in Spring is the Jakarta Commons Logging API (JCL). We
compile against JCL and we also make JCL Log objects visible for classes that extend
the Spring Framework. It’s important to users that all versions of Spring use the same
logging library: migration is easy because backwards compatibility is preserved even
with applications that extend Spring. The way we do this is to make one of the modules
in Spring depend explicitly on commons-logging (the canonical implementation of JCL),
and then make all the other modules depend on that at compile time. If you are using
Maven for example, and wondering where you picked up the dependency on commons-logging,
then it is from Spring and specifically from the central module called spring-core.

The nice thing about commons-logging is that you don’t need anything else to make your
application work. It has a runtime discovery algorithm that looks for other logging
frameworks in well known places on the classpath and uses one that it thinks is
appropriate (or you can tell it which one if you need to). If nothing else is available
you get pretty nice looking logs just from the JDK (java.util.logging or JUL for short).
You should find that your Spring application works and logs happily to the console out
of the box in most situations, and that’s important.

Using Log4j 1.2 or 2.x

Many people use Log4j as a logging framework for
configuration and management purposes. It is efficient and well-established, and in
fact it is what we use at runtime when we build Spring. Spring also provides some
utilities for configuring and initializing Log4j, so it has an optional compile-time
dependency on Log4j in some modules.

To make Log4j 1.2 work with the default JCL dependency (commons-logging) all you
need to do is put Log4j on the classpath, and provide it with a configuration file
(log4j.properties or log4j.xml in the root of the classpath). So for Maven users
this is your dependency declaration:

To use Log4j 2.x with JCL, all you need to do is put Log4j on the classpath and
provide it with a configuration file (log4j2.xml, log4j2.properties, or other
supported configuration
formats). For Maven users, the minimal dependencies needed are:

Avoiding Commons Logging

Unfortunately, the runtime discovery algorithm in the standard commons-logging API,
while convenient for the end-user, can be problematic. If you’d like to avoid JCL’s
standard lookup, there are basically two ways to switch it off:

Exclude the dependency from the spring-core module (as it is the only module that
explicitly depends on commons-logging)

Depend on a special commons-logging dependency that replaces the library with
an empty jar (more details can be found in the
SLF4J FAQ)

To exclude commons-logging, add the following to your dependencyManagement section:

Now this application is currently broken because there is no implementation of the JCL
API on the classpath, so to fix it a new one has to be provided. In the next section we
show you how to provide an alternative implementation of JCL using SLF4J.

Using SLF4J with Log4j or Logback

The Simple Logging Facade for Java (SLF4J) is a popular API
used by other libraries commonly used with Spring. It is typically used with
Logback which is a native implementation of the SLF4J API.

SLF4J provides bindings to many common logging frameworks, including Log4j, and it also
does the reverse: bridges between other logging frameworks and itself. So to use SLF4J
with Spring you need to replace the commons-logging dependency with the SLF4J-JCL
bridge. Once you have done that then logging calls from within Spring will be translated
into logging calls to the SLF4J API, so if other libraries in your application use that
API, then you have a single place to configure and manage logging.

A common choice might be to bridge Spring to SLF4J, and then provide explicit binding
from SLF4J to Log4j. You need to supply several dependencies (and exclude the existing
commons-logging): the JCL bridge, the SLF4j binding to Log4j, and the Log4j provider
itself. In Maven you would do that like this

A more common choice amongst SLF4J users, which uses fewer steps and generates fewer
dependencies, is to bind directly to Logback. This removes the
extra binding step because Logback implements SLF4J directly, so you only need to depend
on just two libraries, namely jcl-over-slf4j and logback):

Using JUL (java.util.logging)

Commons Logging will delegate to java.util.logging by default, provided that no
Log4j is detected on the classpath. So there is no special dependency to set up:
just use Spring with no external dependency for log output to java.util.logging,
either in a standalone application (with a custom or default JUL setup at the JDK
level) or with an application server’s log system (and its system-wide JUL setup).

Commons Logging on WebSphere

Spring applications may run on a container that itself provides an implementation of
JCL, e.g. IBM’s WebSphere Application Server (WAS). This does not cause issues per se
but leads to two different scenarios that need to be understood:

In a "parent first" ClassLoader delegation model (the default on WAS), applications
will always pick up the server-provided version of Commons Logging, delegating to the
WAS logging subsystem (which is actually based on JUL). An application-provided variant
of JCL, whether standard Commons Logging or the JCL-over-SLF4J bridge, will effectively
be ignored, along with any locally included log provider.

With a "parent last" delegation model (the default in a regular Servlet container but
an explicit configuration option on WAS), an application-provided Commons Logging
variant will be picked up, enabling you to set up a locally included log provider,
e.g. Log4j or Logback, within your application. In case of no local log provider,
regular Commons Logging will delegate to JUL by default, effectively logging to
WebSphere’s logging subsystem like in the "parent first" scenario.

All in all, we recommend deploying Spring applications in the "parent last" model
since it naturally allows for local providers as well as the server’s log subsystem.

Part II. What’s New in Spring Framework 4.x

This chapter provides an overview of the new features and improvements that have been introduced with Spring Framework 4.3. If you are interested in more details, please see the Issue Tracker tickets that were resolved as part of the 4.3 development process.

3. New Features and Enhancements in Spring Framework 4.0

The Spring Framework was first released in 2004; since then there have been significant
major revisions: Spring 2.0 provided XML namespaces and AspectJ support; Spring 2.5
embraced annotation-driven configuration; Spring 3.0 introduced a strong Java 5+ foundation
across the framework codebase, and features such as the Java-based @Configuration model.

Version 4.0 is the latest major release of the Spring Framework and the first to fully
support Java 8 features. You can still use Spring with older versions of Java, however,
the minimum requirement has now been raised to Java SE 6. We have also taken the
opportunity of a major release to remove many deprecated classes and methods.

3.1 Improved Getting Started Experience

The new spring.io website provides a whole series of
"Getting Started" guides to help you learn Spring. You
can read more about the guides in the Chapter 1, Getting Started with Spring section
in this document. The new website also provides a comprehensive overview of the many
additional projects that are released under the Spring umbrella.

If you are a Maven user you may also be interested in the helpful
bill of materials POM file that is now published with each Spring
Framework release.

3.2 Removed Deprecated Packages and Methods

All deprecated packages, and many deprecated classes and methods have been removed with
version 4.0. If you are upgrading from a previous release of Spring, you should ensure
that you have fixed any deprecated calls that you were making to outdated APIs.

Note that optional third-party dependencies have been raised to a 2010/2011 minimum
(i.e. Spring 4 generally only supports versions released in late 2010 or later now):
notably, Hibernate 3.6+, EhCache 2.1+, Quartz 1.8+, Groovy 1.8+, and Joda-Time 2.0+.
As an exception to the rule, Spring 4 requires the recent Hibernate Validator 4.3+,
and support for Jackson has been focused on 2.0+ now (with Jackson 1.8/1.9 support
retained for the time being where Spring 3.2 had it; now just in deprecated form).

3.3 Java 8 (as well as 6 and 7)

Spring Framework 4.0 provides support for several Java 8 features. You can make use of
lambda expressions and method references with Spring’s callback interfaces. There
is first-class support for java.time (JSR-310),
and several existing annotations have been retrofitted as @Repeatable. You can also
use Java 8’s parameter name discovery (based on the -parameters compiler flag) as an
alternative to compiling your code with debug information enabled.

Spring remains compatible with older versions of Java and the JDK: concretely, Java SE 6
(specifically, a minimum level equivalent to JDK 6 update 18, as released in January 2010)
and above are still fully supported. However, for newly started development projects
based on Spring 4, we recommend the use of Java 7 or 8.

3.4 Java EE 6 and 7

Java EE version 6 or above is now considered the baseline for Spring Framework 4, with
the JPA 2.0 and Servlet 3.0 specifications being of particular relevance. In order to
remain compatible with Google App Engine and older application servers, it is possible
to deploy a Spring 4 application into a Servlet 2.5 environment. However, Servlet 3.0+
is strongly recommended and a prerequisite in Spring’s test and mock packages for test
setups in development environments.

Note

If you are a WebSphere 7 user, be sure to install the JPA 2.0 feature pack. On
WebLogic 10.3.4 or higher, install the JPA 2.0 patch that comes with it. This turns
both of those server generations into Spring 4 compatible deployment environments.

On a more forward-looking note, Spring Framework 4.0 supports the Java EE 7 level of
applicable specifications now: in particular, JMS 2.0, JTA 1.2, JPA 2.1, Bean Validation
1.1, and JSR-236 Concurrency Utilities. As usual, this support focuses on individual
use of those specifications, e.g. on Tomcat or in standalone environments. However,
it works equally well when a Spring application is deployed to a Java EE 7 server.

Note that Hibernate 4.3 is a JPA 2.1 provider and therefore only supported as of
Spring Framework 4.0. The same applies to Hibernate Validator 5.0 as a Bean Validation
1.1 provider. Neither of the two are officially supported with Spring Framework 3.2.

3.5 Groovy Bean Definition DSL

Beginning with Spring Framework 4.0, it is possible to define external bean configuration
using a Groovy DSL. This is similar in concept to using XML bean definitions but allows
for a more concise syntax. Using Groovy also allows you to easily embed bean definitions
directly in your bootstrap code. For example:

3.6 Core Container Improvements

There have been several general improvements to the core container:

Spring now treats generic types as a form of
qualifier when injecting Beans. For example, if you are using a Spring Data
Repository you can now easily inject a specific implementation:
@Autowired Repository<Customer> customerRepository.

A generalized model for conditionally filtering beans has
been added via the @Conditional annotation. This is similar to @Profile support but
allows for user-defined strategies to be developed programmatically.

CGLIB-based proxy classes no longer require a default
constructor. Support is provided via the objenesis
library which is repackaged inline and distributed as part of the Spring Framework.
With this strategy, no constructor at all is being invoked for proxy instances anymore.

There is managed time zone support across the framework now, e.g. on LocaleContext.

3.7 General Web Improvements

Deployment to Servlet 2.5 servers remains an option, but Spring Framework 4.0 is now
focused primarily on Servlet 3.0+ environments. If you are using the
Spring MVC Test Framework you
will need to ensure that a Servlet 3.0 compatible JAR is in your test classpath.

In addition to the WebSocket support mentioned later, the following general improvements
have been made to Spring’s Web modules:

You can use the new @RestController annotation with Spring
MVC applications, removing the need to add @ResponseBody to each of your
@RequestMapping methods.

3.8 WebSocket, SockJS, and STOMP Messaging

A new spring-websocket module provides comprehensive support for WebSocket-based,
two-way communication between client and server in web applications. It is compatible with
JSR-356, the Java WebSocket API, and in addition
provides SockJS-based fallback options (i.e. WebSocket emulation) for use in browsers
that don’t yet support the WebSocket protocol (e.g. Internet Explorer < 10).

A new spring-messaging module adds support for STOMP as the WebSocket sub-protocol
to use in applications along with an annotation programming model for routing and
processing STOMP messages from WebSocket clients. As a result an @Controller
can now contain both @RequestMapping and @MessageMapping methods for handling
HTTP requests and messages from WebSocket-connected clients. The new spring-messaging
module also contains key abstractions formerly from the
Spring Integration project such as
Message, MessageChannel, MessageHandler, and others to serve as a foundation
for messaging-based applications.

3.9 Testing Improvements

In addition to pruning of deprecated code within the spring-test module, Spring
Framework 4.0 introduces several new features for use in unit and integration testing.

Almost all annotations in the spring-test module (e.g., @ContextConfiguration,
@WebAppConfiguration, @ContextHierarchy, @ActiveProfiles, etc.) can now be used
as meta-annotations to create custom
composed annotations and reduce configuration duplication across a test suite.

Active bean definition profiles can now be resolved programmatically, simply by
implementing a custom ActiveProfilesResolver
and registering it via the resolver attribute of @ActiveProfiles.

A new SocketUtils class has been introduced in the spring-core module
which enables you to scan for free TCP and UDP server ports on localhost. This
functionality is not specific to testing but can prove very useful when writing
integration tests that require the use of sockets, for example tests that start
an in-memory SMTP server, FTP server, Servlet container, etc.

As of Spring 4.0, the set of mocks in the org.springframework.mock.web package is
now based on the Servlet 3.0 API. Furthermore, several of the Servlet API mocks
(e.g., MockHttpServletRequest, MockServletContext, etc.) have been updated with
minor enhancements and improved configurability.

4. New Features and Enhancements in Spring Framework 4.1

Version 4.1 included a number of improvements, as described in the following sections:

4.1 JMS Improvements

Spring 4.1 introduces a much simpler infrastructure to register JMS
listener endpoints by annotating bean methods with
@JmsListener.
The XML namespace has been enhanced to support this new style (jms:annotation-driven),
and it is also possible to fully configure the infrastructure using Java config
(@EnableJms,
JmsListenerContainerFactory). It is also possible to register listener endpoints
programmatically using
JmsListenerConfigurer.

Spring 4.1 also aligns its JMS support to allow you to benefit from the spring-messaging
abstraction introduced in 4.0, that is:

Message listener endpoints can have a more flexible signature and benefit from
standard messaging annotations such as @Payload, @Header, @Headers, and @SendTo. It
is also possible to use a standard Message in lieu of javax.jms.Message as method
argument.

A new JmsMessageOperations
interface is available and permits JmsTemplate like operations using the Message
abstraction.

Finally, Spring 4.1 provides additional miscellaneous improvements:

Synchronous request-reply operations support in JmsTemplate

Listener priority can be specified per <jms:listener/> element

Recovery options for the message listener container are configurable using a
BackOff implementation

JMS 2.0 shared consumers are supported

4.2 Caching Improvements

Spring 4.1 supports JCache (JSR-107) annotations using Spring’s
existing cache configuration and infrastructure abstraction; no changes are required
to use the standard annotations.

Spring 4.1 also improves its own caching abstraction significantly:

Caches can be resolved at runtime using a
CacheResolver. As a result the
value argument defining the cache name(s) to use is no longer mandatory.

Spring 4.1 also has a breaking change in the Cache interface as a new putIfAbsent
method has been added.

4.3 Web Improvements

The existing support for resource handling based on the ResourceHttpRequestHandler
has been expanded with new abstractions ResourceResolver, ResourceTransformer,
and ResourceUrlProvider. A number of built-in implementations provide support
for versioned resource URLs (for effective HTTP caching), locating gzipped resources,
generating an HTML 5 AppCache manifests, and more. See Section 22.16.9, “Serving of Resources”.

ListenableFuture is supported as a return value alternative to DeferredResult
where an underlying service (or perhaps a call to AsyncRestTemplate) already
returns ListenableFuture.

@ModelAttribute methods are now invoked in an order that respects inter-dependencies.
See SPR-6299.

Jackson’s @JsonView is supported directly on @ResponseBody and ResponseEntity
controller methods for serializing different amounts of detail for the same POJO (e.g.
summary vs. detail page). This is also supported with View-based rendering by
adding the serialization view type as a model attribute under a special key.
See the section called “Jackson Serialization View Support” for details.

A new lifecycle option is available for intercepting @ResponseBody and ResponseEntity
methods just after the controller method returns and before the response is written.
To take advantage declare an @ControllerAdvice bean that implements ResponseBodyAdvice.
The built-in support for @JsonView and JSONP take advantage of this.
See Section 22.4.1, “Intercepting requests with a HandlerInterceptor”.

There are three new HttpMessageConverter options:

Gson — lighter footprint than Jackson; has already been in use in Spring Android.

Google Protocol Buffers — efficient and effective as an inter-service communication
data protocol within an enterprise but can also be exposed as JSON and XML for browsers.

Jackson based XML serialization is now supported through the
jackson-dataformat-xml extension.
When using @EnableWebMvc or <mvc:annotation-driven/>, this is used by default
instead of JAXB2 if jackson-dataformat-xml is in the classpath.

Views such as JSPs can now build links to controllers by referring to controller mappings
by name. A default name is assigned to every @RequestMapping. For example FooController
with method handleFoo is named "FC#handleFoo". The naming strategy is pluggable.
It is also possible to name an @RequestMapping explicitly through its name attribute.
A new mvcUrl function in the Spring JSP tag library makes this easy to use in JSP pages.
See Section 22.7.3, “Building URIs to Controllers and methods from views”.

View controllers now have built-in support for redirects and for setting the response
status. An application can use this to configure redirect URLs, render 404 responses
with a view, send "no content" responses, etc.
Some use cases are
listed here.

Various improvements to MockServletContext, MockHttpServletRequest, and other
Servlet API mocks.

AssertThrows has been refactored to support Throwable instead of Exception.

In Spring MVC Test, JSON responses can be asserted with JSON Assert
as an extra option to using JSONPath much like it has been possible to do for XML with
XMLUnit.

MockMvcBuilderrecipes can now be created with the help of MockMvcConfigurer. This
was added to make it easy to apply Spring Security setup but can be used to encapsulate
common setup for any 3rd party framework or within a project.

MockRestServiceServer now supports the AsyncRestTemplate for client-side testing.

5. New Features and Enhancements in Spring Framework 4.2

Version 4.2 included a number of improvements, as described in the following sections:

Configuration classes may declare an @Order value, getting processed in a corresponding
order (e.g. for overriding beans by name) even when detected through classpath scanning.

@Resource injection points support an @Lazy declaration, analogous to @Autowired,
receiving a lazy-initializing proxy for the requested target bean.

The application event infrastructure now offers an annotation-based model as well as the ability to publish any arbitrary event.

Any public method in a managed bean can be annotated with @EventListener to consume events.

@TransactionalEventListener provides transaction-bound event support.

Spring Framework 4.2 introduces first-class support for declaring and
looking up aliases for annotation attributes. The new @AliasFor
annotation can be used to declare a pair of aliased attributes within
a single annotation or to declare an alias from one attribute in a
custom composed annotation to an attribute in a meta-annotation.

Similarly, composed annotations that override attributes from
meta-annotations can now use @AliasFor for fine-grained control
over exactly which attributes are overridden within an annotation
hierarchy. In fact, it is now possible to declare an alias for the
value attribute of a meta-annotation.

For example, one can now develop a composed annotation with a custom
attribute override as follows.

Numerous improvements to Spring’s search algorithms used for finding
meta-annotations. For example, locally declared composed annotations
are now favored over inherited annotations.

Composed annotations that override attributes from meta-annotations
can now be discovered on interfaces and on abstract, bridge, & interface
methods as well as on classes, standard methods, constructors, and
fields.

The features of field-based data binding (DirectFieldAccessor) have been aligned with the current
property-based data binding (BeanWrapper). In particular, field-based binding now supports
navigation for Collections, Arrays, and Maps.

DefaultConversionService now provides out-of-the-box converters for Stream, Charset,
Currency, and TimeZone. Such converters can be added individually to any arbitrary
ConversionService as well.

DefaultFormattingConversionService comes with out-of-the-box support for the value types
in JSR-354 Money & Currency (if the 'javax.money' API is present on the classpath): namely,
MonetaryAmount and CurrencyUnit. This includes support for applying @NumberFormat.

@NumberFormat can now be used as a meta-annotation.

JavaMailSenderImpl has a new testConnection() method for checking connectivity to the server.

ScheduledTaskRegistrar exposes scheduled tasks.

Apache commons-pool2 is now supported for a pooling AOP CommonsPool2TargetSource.

Introduced StandardScriptFactory as a JSR-223 based mechanism for scripted beans,
exposed through the lang:std element in XML. Supports e.g. JavaScript and JRuby.
(Note: JRubyScriptFactory and lang:jruby are deprecated now, in favor of using JSR-223.)

5.2 Data Access Improvements

javax.transaction.Transactional is now supported via AspectJ.

SimpleJdbcCallOperations now supports named binding.

Full support for Hibernate ORM 5.0: as a JPA provider (automatically adapted) as well as
through its native API (covered by the new org.springframework.orm.hibernate5 package).

Embedded databases can now be automatically assigned unique names, and
<jdbc:embedded-database> supports a new database-name attribute.
See "Testing Improvements" below for further details.

5.3 JMS Improvements

The autoStartup attribute can be controlled via JmsListenerContainerFactory.

The type of the reply Destination can now be configured per listener container.

ListenableFuture and CompletableFuture as return value types from
@MessageMapping and @SubscribeMapping methods.

MarshallingMessageConverter for XML payloads.

5.6 Testing Improvements

JUnit-based integration tests can now be executed with JUnit rules instead of the
SpringJUnit4ClassRunner. This allows Spring-based integration tests to be run with
alternative runners like JUnit’s Parameterized or third-party runners such as the
MockitoJUnitRunner.

The Spring MVC Test framework now provides first-class support for HtmlUnit,
including integration with Selenium’s WebDriver, allowing for page-based
web application testing without the need to deploy to a Servlet container.

ReflectionTestUtils now supports setting and getting static fields,
including constants.

The original ordering of bean definition profiles declared via
@ActiveProfiles is now retained in order to support use cases such
as Spring Boot’s ConfigFileApplicationListener which loads
configuration files based on the names of active profiles.

@DirtiesContext supports new BEFORE_METHOD, BEFORE_CLASS, and
BEFORE_EACH_TEST_METHOD modes for closing the ApplicationContextbefore a test — for example, if some rogue (i.e., yet to be
determined) test within a large test suite has corrupted the original
configuration for the ApplicationContext.

@Commit is a new annotation that may be used as a direct replacement for
@Rollback(false).

@Rollback may now be used to configure class-level default rollback semantics.

Consequently, @TransactionConfiguration is now deprecated and will be removed in a
subsequent release.

@Sql now supports execution of inlined SQL statements via a new
statements attribute.

The ContextCache that is used for caching ApplicationContexts
between tests is now a public API with a default implementation that
can be replaced for custom caching needs.

DefaultTestContext, DefaultBootstrapContext, and
DefaultCacheAwareContextLoaderDelegate are now public classes in the
support subpackage, allowing for custom extensions.

TestContextBootstrappers are now responsible for building the
TestContext.

In the Spring MVC Test framework, MvcResult details can now be logged
at DEBUG level or written to a custom OutputStream or Writer. See
the new log(), print(OutputStream), and print(Writer) methods in
MockMvcResultHandlers for details.

The JDBC XML namespace supports a new database-name attribute in
<jdbc:embedded-database>, allowing developers to set unique names
for embedded databases –- for example, via a SpEL expression or a
property placeholder that is influenced by the current active bean
definition profiles.

Embedded databases can now be automatically assigned a unique name,
allowing common test database configuration to be reused in different
ApplicationContexts within a test suite.

6.1 Core Container Improvements

Lazy candidate beans are not being created in case of injecting a primary bean.

It is no longer necessary to specify the @Autowired annotation if the target
bean only defines one constructor.

@Configuration classes support constructor injection.

Any SpEL expression used to specify the condition of an @EventListener can
now refer to beans (e.g. @beanName.method()).

Composed annotations can now override array attributes in meta-annotations
with a single element of the component type of the array. For example, the
String[] path attribute of @RequestMapping can be overridden with
String path in a composed annotation.

@PersistenceContext/@PersistenceUnit selects a primary EntityManagerFactory
bean if declared as such.

@Scheduled and @Schedules may now be used as meta-annotations to create
custom composed annotations with attribute overrides.

@Scheduled is properly supported on beans of any scope.

6.2 Data Access Improvements

jdbc:initialize-database and jdbc:embedded-database support a configurable
separator to be applied to each script.

6.3 Caching Improvements

Spring 4.3 allows concurrent calls on a given key to be synchronized so that the
value is only computed once. This is an opt-in feature that should be enabled via
the new sync attribute on @Cacheable. This features introduces a breaking
change in the Cache interface as a get(Object key, Callable<T> valueLoader)
method has been added.

Spring 4.3 also improves the caching abstraction as follows:

SpEL expressions in caches-related annotations can now refer to beans (i.e.
@beanName.method()).

ConcurrentMapCacheManager and ConcurrentMapCache now support the serialization
of cache entries via a new storeByValue attribute.

@Cacheable, @CacheEvict, @CachePut, and @Caching may now be used as
meta-annotations to create custom composed annotations with attribute overrides.

6.4 JMS Improvements

@SendTo can now be specified at the class level to share a common reply destination.

@JmsListener and @JmsListeners may now be used as meta-annotations to create
custom composed annotations with attribute overrides.

6.6 WebSocket Messaging Improvements

@SendTo and @SendToUser can now be specified at class-level to share a common destination.

6.7 Testing Improvements

The JUnit support in the Spring TestContext Framework now requires JUnit 4.12 or higher.

New SpringRunneralias for the SpringJUnit4ClassRunner.

Test related annotations may now be declared on interfaces — for example, for use with
test interfaces that make use of Java 8 based interface default methods.

An empty declaration of @ContextConfiguration can now be completely omitted if default
XML files, Groovy scripts, or @Configuration classes are detected.

@Transactional test methods are no longer required to be public (e.g., in TestNG and JUnit 5).

@BeforeTransaction and @AfterTransaction methods are no longer required to be public
and may now be declared on Java 8 based interface default methods.

The ApplicationContext cache in the Spring TestContext Framework is now bounded with a
default maximum size of 32 and a least recently used eviction policy. The maximum size
can be configured by setting a JVM system property or Spring property called
spring.test.context.cache.maxSize.

New ContextCustomizer API for customizing a test ApplicationContextafter bean
definitions have been loaded into the context but before the context has been refreshed.
Customizers can be registered globally by third parties, foregoing the need to implement a
custom ContextLoader.

@Sql and @SqlGroup may now be used as meta-annotations to create custom composed
annotations with attribute overrides.

ReflectionTestUtils now automatically unwraps proxies when setting or getting a field.

Client-side REST test support allows indicating how many times a request is expected and
whether the order of declaration for expectations should be ignored (see Section 15.6.3, “Client-Side REST Tests”).

Client-side REST Test supports expectations for form data in the request body.

Part III. Core Technologies

This part of the reference documentation covers all of those technologies that are
absolutely integral to the Spring Framework.

Foremost amongst these is the Spring Framework’s Inversion of Control (IoC) container. A
thorough treatment of the Spring Framework’s IoC container is closely followed by
comprehensive coverage of Spring’s Aspect-Oriented Programming (AOP) technologies. The
Spring Framework has its own AOP framework, which is conceptually easy to understand,
and which successfully addresses the 80% sweet spot of AOP requirements in Java
enterprise programming.

Coverage of Spring’s integration with AspectJ (currently the richest - in terms of
features - and certainly most mature AOP implementation in the Java enterprise space) is
also provided.

7. The IoC container

7.1 Introduction to the Spring IoC container and beans

This chapter covers the Spring Framework implementation of the Inversion of Control
(IoC) [1] principle. IoC
is also known as dependency injection (DI). It is a process whereby objects define
their dependencies, that is, the other objects they work with, only through constructor
arguments, arguments to a factory method, or properties that are set on the object
instance after it is constructed or returned from a factory method. The container then
injects those dependencies when it creates the bean. This process is fundamentally
the inverse, hence the name Inversion of Control (IoC), of the bean itself
controlling the instantiation or location of its dependencies by using direct
construction of classes, or a mechanism such as the Service Locator pattern.

The org.springframework.beans and org.springframework.context packages are the basis
for Spring Framework’s IoC container. The
BeanFactory
interface provides an advanced configuration mechanism capable of managing any type of
object.
ApplicationContext
is a sub-interface of BeanFactory. It adds easier integration with Spring’s AOP
features; message resource handling (for use in internationalization), event
publication; and application-layer specific contexts such as the WebApplicationContext
for use in web applications.

In short, the BeanFactory provides the configuration framework and basic
functionality, and the ApplicationContext adds more enterprise-specific functionality.
The ApplicationContext is a complete superset of the BeanFactory, and is used
exclusively in this chapter in descriptions of Spring’s IoC container. For more
information on using the BeanFactory instead of the ApplicationContext, refer to
Section 7.16, “The BeanFactory”.

In Spring, the objects that form the backbone of your application and that are managed
by the Spring IoC container are called beans. A bean is an object that is
instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a
bean is simply one of many objects in your application. Beans, and the dependencies
among them, are reflected in the configuration metadata used by a container.

7.2 Container overview

The interface org.springframework.context.ApplicationContext represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the
aforementioned beans. The container gets its instructions on what objects to
instantiate, configure, and assemble by reading configuration metadata. The
configuration metadata is represented in XML, Java annotations, or Java code. It allows
you to express the objects that compose your application and the rich interdependencies
between such objects.

Several implementations of the ApplicationContext interface are supplied
out-of-the-box with Spring. In standalone applications it is common to create an
instance of
ClassPathXmlApplicationContext
or FileSystemXmlApplicationContext.
While XML has been the traditional format for defining configuration metadata you can
instruct the container to use Java annotations or code as the metadata format by
providing a small amount of XML configuration to declaratively enable support for these
additional metadata formats.

In most application scenarios, explicit user code is not required to instantiate one or
more instances of a Spring IoC container. For example, in a web application scenario, a
simple eight (or so) lines of boilerplate web descriptor XML in the web.xml file
of the application will typically suffice (see Section 7.15.4, “Convenient ApplicationContext instantiation for web applications”). If you are using the
Spring Tool Suite Eclipse-powered development
environment this boilerplate configuration can be easily created with few mouse clicks or
keystrokes.

The following diagram is a high-level view of how Spring works. Your application classes
are combined with configuration metadata so that after the ApplicationContext is
created and initialized, you have a fully configured and executable system or
application.

Figure 7.1. The Spring IoC container

7.2.1 Configuration metadata

As the preceding diagram shows, the Spring IoC container consumes a form of
configuration metadata; this configuration metadata represents how you as an
application developer tell the Spring container to instantiate, configure, and assemble
the objects in your application.

Configuration metadata is traditionally supplied in a simple and intuitive XML format,
which is what most of this chapter uses to convey key concepts and features of the
Spring IoC container.

Note

XML-based metadata is not the only allowed form of configuration metadata. The
Spring IoC container itself is totally decoupled from the format in which this
configuration metadata is actually written. These days many developers choose
Java-based configuration for their Spring applications.

For information about using other forms of metadata with the Spring container, see:

Java-based configuration: Starting with Spring 3.0, many features
provided by the Spring JavaConfig project became part of the core Spring Framework.
Thus you can define beans external to your application classes by using Java rather
than XML files. To use these new features, see the @Configuration, @Bean, @Import
and @DependsOn annotations.

Spring configuration consists of at least one and typically more than one bean
definition that the container must manage. XML-based configuration metadata shows these
beans configured as <bean/> elements inside a top-level <beans/> element. Java
configuration typically uses @Bean annotated methods within a @Configuration class.

These bean definitions correspond to the actual objects that make up your application.
Typically you define service layer objects, data access objects (DAOs), presentation
objects such as Struts Action instances, infrastructure objects such as Hibernate
SessionFactories, JMS Queues, and so forth. Typically one does not configure
fine-grained domain objects in the container, because it is usually the responsibility
of DAOs and business logic to create and load domain objects. However, you can use
Spring’s integration with AspectJ to configure objects that have been created outside
the control of an IoC container. See Using AspectJ to
dependency-inject domain objects with Spring.

The following example shows the basic structure of XML-based configuration metadata:

The id attribute is a string that you use to identify the individual bean definition.
The class attribute defines the type of the bean and uses the fully qualified
classname. The value of the id attribute refers to collaborating objects. The XML for
referring to collaborating objects is not shown in this example; see
Dependencies for more information.

7.2.2 Instantiating a container

Instantiating a Spring IoC container is straightforward. The location path or paths
supplied to an ApplicationContext constructor are actually resource strings that allow
the container to load configuration metadata from a variety of external resources such
as the local file system, from the Java CLASSPATH, and so on.

After you learn about Spring’s IoC container, you may want to know more about Spring’s
Resource abstraction, as described in Chapter 8, Resources, which provides a convenient
mechanism for reading an InputStream from locations defined in a URI syntax. In
particular, Resource paths are used to construct applications contexts as described in
Section 8.7, “Application contexts and Resource paths”.

The following example shows the service layer objects (services.xml) configuration file:

In the preceding example, the service layer consists of the class PetStoreServiceImpl,
and two data access objects of the type JpaAccountDao and JpaItemDao (based
on the JPA Object/Relational mapping standard). The property name element refers to the
name of the JavaBean property, and the ref element refers to the name of another bean
definition. This linkage between id and ref elements expresses the dependency between
collaborating objects. For details of configuring an object’s dependencies, see
Dependencies.

Composing XML-based configuration metadata

It can be useful to have bean definitions span multiple XML files. Often each individual
XML configuration file represents a logical layer or module in your architecture.

You can use the application context constructor to load bean definitions from all these
XML fragments. This constructor takes multiple Resource locations, as was shown in the
previous section. Alternatively, use one or more occurrences of the <import/> element
to load bean definitions from another file or files. For example:

In the preceding example, external bean definitions are loaded from three files:
services.xml, messageSource.xml, and themeSource.xml. All location paths are
relative to the definition file doing the importing, so services.xml must be in the
same directory or classpath location as the file doing the importing, while
messageSource.xml and themeSource.xml must be in a resources location below the
location of the importing file. As you can see, a leading slash is ignored, but given
that these paths are relative, it is better form not to use the slash at all. The
contents of the files being imported, including the top level <beans/> element, must
be valid XML bean definitions according to the Spring Schema.

Note

It is possible, but not recommended, to reference files in parent directories using a
relative "../" path. Doing so creates a dependency on a file that is outside the current
application. In particular, this reference is not recommended for "classpath:" URLs (for
example, "classpath:../services.xml"), where the runtime resolution process chooses the
"nearest" classpath root and then looks into its parent directory. Classpath
configuration changes may lead to the choice of a different, incorrect directory.

You can always use fully qualified resource locations instead of relative paths: for
example, "file:C:/config/services.xml" or "classpath:/config/services.xml". However, be
aware that you are coupling your application’s configuration to specific absolute
locations. It is generally preferable to keep an indirection for such absolute
locations, for example, through "${…​}" placeholders that are resolved against JVM
system properties at runtime.

The import directive is a feature provided by the beans namespace itself. Further
configuration features beyond plain bean definitions are available in a selection
of XML namespaces provided by Spring, e.g. the "context" and the "util" namespace.

The Groovy Bean Definition DSL

As a further example for externalized configuration metadata, bean definitions can also
be expressed in Spring’s Groovy Bean Definition DSL, as known from the Grails framework.
Typically, such configuration will live in a ".groovy" file with a structure as follows:

This configuration style is largely equivalent to XML bean definitions and even
supports Spring’s XML configuration namespaces. It also allows for importing XML
bean definition files through an "importBeans" directive.

7.2.3 Using the container

The ApplicationContext is the interface for an advanced factory capable of maintaining
a registry of different beans and their dependencies. Using the method T getBean(String
name, Class<T> requiredType) you can retrieve instances of your beans.

The ApplicationContext enables you to read bean definitions and access them as follows:

Such reader delegates can be mixed and matched on the same ApplicationContext,
reading bean definitions from diverse configuration sources, if desired.

You can then use getBean to retrieve instances of your beans. The ApplicationContext
interface has a few other methods for retrieving beans, but ideally your application
code should never use them. Indeed, your application code should have no calls to the
getBean() method at all, and thus no dependency on Spring APIs at all. For example,
Spring’s integration with web frameworks provides dependency injection for various web
framework components such as controllers and JSF-managed beans, allowing you to declare
a dependency on a specific bean through metadata (e.g. an autowiring annotation).

7.3 Bean overview

A Spring IoC container manages one or more beans. These beans are created with the
configuration metadata that you supply to the container, for example, in the form of XML
<bean/> definitions.

Within the container itself, these bean definitions are represented as BeanDefinition
objects, which contain (among other information) the following metadata:

A package-qualified class name: typically the actual implementation class of the
bean being defined.

Bean behavioral configuration elements, which state how the bean should behave in the
container (scope, lifecycle callbacks, and so forth).

References to other beans that are needed for the bean to do its work; these
references are also called collaborators or dependencies.

Other configuration settings to set in the newly created object, for example, the
number of connections to use in a bean that manages a connection pool, or the size
limit of the pool.

This metadata translates to a set of properties that make up each bean definition.

In addition to bean definitions that contain information on how to create a specific
bean, the ApplicationContext implementations also permit the registration of existing
objects that are created outside the container, by users. This is done by accessing the
ApplicationContext’s BeanFactory via the method getBeanFactory() which returns the
BeanFactory implementation DefaultListableBeanFactory. DefaultListableBeanFactory
supports this registration through the methods registerSingleton(..) and
registerBeanDefinition(..). However, typical applications work solely with beans
defined through metadata bean definitions.

Note

Bean metadata and manually supplied singleton instances need to be registered as early
as possible, in order for the container to properly reason about them during autowiring
and other introspection steps. While overriding of existing metadata and existing
singleton instances is supported to some degree, the registration of new beans at
runtime (concurrently with live access to factory) is not officially supported and may
lead to concurrent access exceptions and/or inconsistent state in the bean container.

7.3.1 Naming beans

Every bean has one or more identifiers. These identifiers must be unique within the
container that hosts the bean. A bean usually has only one identifier, but if it
requires more than one, the extra ones can be considered aliases.

In XML-based configuration metadata, you use the id and/or name attributes
to specify the bean identifier(s). The id attribute allows you to specify
exactly one id. Conventionally these names are alphanumeric ('myBean',
'fooService', etc.), but may contain special characters as well. If you want to
introduce other aliases to the bean, you can also specify them in the name
attribute, separated by a comma (,), semicolon (;), or white space. As a
historical note, in versions prior to Spring 3.1, the id attribute was
defined as an xsd:ID type, which constrained possible characters. As of 3.1,
it is defined as an xsd:string type. Note that bean id uniqueness is still
enforced by the container, though no longer by XML parsers.

You are not required to supply a name or id for a bean. If no name or id is supplied
explicitly, the container generates a unique name for that bean. However, if you want to
refer to that bean by name, through the use of the ref element or
Service Locator style lookup, you must provide a name.
Motivations for not supplying a name are related to using inner
beans and autowiring collaborators.

Bean Naming Conventions

The convention is to use the standard Java convention for instance field names when
naming beans. That is, bean names start with a lowercase letter, and are camel-cased
from then on. Examples of such names would be (without quotes) 'accountManager',
'accountService', 'userDao', 'loginController', and so forth.

Naming beans consistently makes your configuration easier to read and understand, and if
you are using Spring AOP it helps a lot when applying advice to a set of beans related
by name.

Note

With component scanning in the classpath, Spring generates bean names for unnamed
components, following the rules above: essentially, taking the simple class name
and turning its initial character to lower-case. However, in the (unusual) special
case when there is more than one character and both the first and second characters
are upper case, the original casing gets preserved. These are the same rules as
defined by java.beans.Introspector.decapitalize (which Spring is using here).

Aliasing a bean outside the bean definition

In a bean definition itself, you can supply more than one name for the bean, by using a
combination of up to one name specified by the id attribute, and any number of other
names in the name attribute. These names can be equivalent aliases to the same bean,
and are useful for some situations, such as allowing each component in an application to
refer to a common dependency by using a bean name that is specific to that component
itself.

Specifying all aliases where the bean is actually defined is not always adequate,
however. It is sometimes desirable to introduce an alias for a bean that is defined
elsewhere. This is commonly the case in large systems where configuration is split
amongst each subsystem, each subsystem having its own set of object definitions. In
XML-based configuration metadata, you can use the <alias/> element to accomplish this.

<aliasname="fromName"alias="toName"/>

In this case, a bean in the same container which is named fromName, may also,
after the use of this alias definition, be referred to as toName.

For example, the configuration metadata for subsystem A may refer to a DataSource via
the name subsystemA-dataSource. The configuration metadata for subsystem B may refer to
a DataSource via the name subsystemB-dataSource. When composing the main application
that uses both these subsystems the main application refers to the DataSource via the
name myApp-dataSource. To have all three names refer to the same object you add to the
MyApp configuration metadata the following aliases definitions:

Now each component and the main application can refer to the dataSource through a name
that is unique and guaranteed not to clash with any other definition (effectively
creating a namespace), yet they refer to the same bean.

7.3.2 Instantiating beans

A bean definition essentially is a recipe for creating one or more objects. The
container looks at the recipe for a named bean when asked, and uses the configuration
metadata encapsulated by that bean definition to create (or acquire) an actual object.

Typically, to specify the bean class to be constructed in the case where the container
itself directly creates the bean by calling its constructor reflectively, somewhat
equivalent to Java code using the new operator.

To specify the actual class containing the static factory method that will be
invoked to create the object, in the less common case where the container invokes a
staticfactory method on a class to create the bean. The object type returned
from the invocation of the static factory method may be the same class or another
class entirely.

Inner class names.
If you want to configure a bean definition for a static nested class, you have to use
the binary name of the nested class.

For example, if you have a class called Foo in the com.example package, and this
Foo class has a static nested class called Bar, the value of the 'class'
attribute on a bean definition would be…​

com.example.Foo$Bar

Notice the use of the $ character in the name to separate the nested class name from
the outer class name.

Instantiation with a constructor

When you create a bean by the constructor approach, all normal classes are usable by and
compatible with Spring. That is, the class being developed does not need to implement
any specific interfaces or to be coded in a specific fashion. Simply specifying the bean
class should suffice. However, depending on what type of IoC you use for that specific
bean, you may need a default (empty) constructor.

The Spring IoC container can manage virtually any class you want it to manage; it is
not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with
only a default (no-argument) constructor and appropriate setters and getters modeled
after the properties in the container. You can also have more exotic non-bean-style
classes in your container. If, for example, you need to use a legacy connection pool
that absolutely does not adhere to the JavaBean specification, Spring can manage it as
well.

With XML-based configuration metadata you can specify your bean class as follows:

For details about the mechanism for supplying arguments to the constructor (if required)
and setting object instance properties after the object is constructed, see
Injecting Dependencies.

Instantiation with a static factory method

When defining a bean that you create with a static factory method, you use the class
attribute to specify the class containing the static factory method and an attribute
named factory-method to specify the name of the factory method itself. You should be
able to call this method (with optional arguments as described later) and return a live
object, which subsequently is treated as if it had been created through a constructor.
One use for such a bean definition is to call static factories in legacy code.

The following bean definition specifies that the bean will be created by calling a
factory-method. The definition does not specify the type (class) of the returned object,
only the class containing the factory method. In this example, the createInstance()
method must be a static method.

For details about the mechanism for supplying (optional) arguments to the factory method
and setting object instance properties after the object is returned from the factory,
see Dependencies and configuration in detail.

Instantiation using an instance factory method

Similar to instantiation through a static
factory method, instantiation with an instance factory method invokes a non-static
method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class attribute empty, and in the factory-bean attribute,
specify the name of a bean in the current (or parent/ancestor) container that contains
the instance method that is to be invoked to create the object. Set the name of the
factory method itself with the factory-method attribute.

<!-- the factory bean, which contains a method called createInstance() --><beanid="serviceLocator"class="examples.DefaultServiceLocator"><!-- inject any dependencies required by this locator bean --></bean><!-- the bean to be created via the factory bean --><beanid="clientService"factory-bean="serviceLocator"factory-method="createClientServiceInstance"/>

In Spring documentation, factory bean refers to a bean that is configured in the
Spring container that will create objects through an
instance or
static factory method. By contrast,
FactoryBean (notice the capitalization) refers to a Spring-specific
FactoryBean.

7.4 Dependencies

A typical enterprise application does not consist of a single object (or bean in the
Spring parlance). Even the simplest application has a few objects that work together to
present what the end-user sees as a coherent application. This next section explains how
you go from defining a number of bean definitions that stand alone to a fully realized
application where objects collaborate to achieve a goal.

7.4.1 Dependency Injection

Dependency injection (DI) is a process whereby objects define their dependencies,
that is, the other objects they work with, only through constructor arguments, arguments
to a factory method, or properties that are set on the object instance after it is
constructed or returned from a factory method. The container then injects those
dependencies when it creates the bean. This process is fundamentally the inverse, hence
the name Inversion of Control (IoC), of the bean itself controlling the instantiation
or location of its dependencies on its own by using direct construction of classes, or
the Service Locator pattern.

Code is cleaner with the DI principle and decoupling is more effective when objects are
provided with their dependencies. The object does not look up its dependencies, and does
not know the location or class of the dependencies. As such, your classes become easier
to test, in particular when the dependencies are on interfaces or abstract base classes,
which allow for stub or mock implementations to be used in unit tests.

Constructor-based dependency injection

Constructor-based DI is accomplished by the container invoking a constructor with a
number of arguments, each representing a dependency. Calling a static factory method
with specific arguments to construct the bean is nearly equivalent, and this discussion
treats arguments to a constructor and to a static factory method similarly. The
following example shows a class that can only be dependency-injected with constructor
injection. Notice that there is nothing special about this class, it is a POJO that
has no dependencies on container specific interfaces, base classes or annotations.

publicclass SimpleMovieLister {
// the SimpleMovieLister has a dependency on a MovieFinderprivate MovieFinder movieFinder;
// a constructor so that the Spring container can inject a MovieFinderpublic SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}

Constructor argument resolution

Constructor argument resolution matching occurs using the argument’s type. If no
potential ambiguity exists in the constructor arguments of a bean definition, then the
order in which the constructor arguments are defined in a bean definition is the order
in which those arguments are supplied to the appropriate constructor when the bean is
being instantiated. Consider the following class:

No potential ambiguity exists, assuming that Bar and Baz classes are not related by
inheritance. Thus the following configuration works fine, and you do not need to specify
the constructor argument indexes and/or types explicitly in the <constructor-arg/>
element.

When another bean is referenced, the type is known, and matching can occur (as was the
case with the preceding example). When a simple type is used, such as
<value>true</value>, Spring cannot determine the type of the value, and so cannot match
by type without help. Consider the following class:

Keep in mind that to make this work out of the box your code must be compiled with the
debug flag enabled so that Spring can look up the parameter name from the constructor.
If you can’t compile your code with debug flag (or don’t want to) you can use
@ConstructorProperties
JDK annotation to explicitly name your constructor arguments. The sample class would
then have to look as follows:

Setter-based dependency injection

The following example shows a class that can only be dependency-injected using pure
setter injection. This class is conventional Java. It is a POJO that has no dependencies
on container specific interfaces, base classes or annotations.

publicclass SimpleMovieLister {
// the SimpleMovieLister has a dependency on the MovieFinderprivate MovieFinder movieFinder;
// a setter method so that the Spring container can inject a MovieFinderpublicvoid setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually uses the injected MovieFinder is omitted...
}

The ApplicationContext supports constructor-based and setter-based DI for the beans it
manages. It also supports setter-based DI after some dependencies have already been
injected through the constructor approach. You configure the dependencies in the form of
a BeanDefinition, which you use in conjunction with PropertyEditor instances to
convert properties from one format to another. However, most Spring users do not work
with these classes directly (i.e., programmatically) but rather with XML bean
definitions, annotated components (i.e., classes annotated with @Component,
@Controller, etc.), or @Bean methods in Java-based @Configuration classes. These
sources are then converted internally into instances of BeanDefinition and used to
load an entire Spring IoC container instance.

Constructor-based or setter-based DI?

Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to
use constructors for mandatory dependencies and setter methods or configuration methods
for optional dependencies. Note that use of the @Required
annotation on a setter method can be used to make the property a required dependency.

The Spring team generally advocates constructor injection as it enables one to implement
application components as immutable objects and to ensure that required dependencies
are not null. Furthermore constructor-injected components are always returned to client
(calling) code in a fully initialized state. As a side note, a large number of constructor
arguments is a bad code smell, implying that the class likely has too many
responsibilities and should be refactored to better address proper separation of concerns.

Setter injection should primarily only be used for optional dependencies that can be
assigned reasonable default values within the class. Otherwise, not-null checks must be
performed everywhere the code uses the dependency. One benefit of setter injection is that
setter methods make objects of that class amenable to reconfiguration or re-injection
later. Management through JMX MBeans is therefore a compelling use case for setter
injection.

Use the DI style that makes the most sense for a particular class. Sometimes, when dealing
with third-party classes for which you do not have the source, the choice is made for you.
For example, if a third-party class does not expose any setter methods, then constructor
injection may be the only available form of DI.

Dependency resolution process

The container performs bean dependency resolution as follows:

The ApplicationContext is created and initialized with configuration metadata that
describes all the beans. Configuration metadata can be specified via XML, Java code, or
annotations.

For each bean, its dependencies are expressed in the form of properties, constructor
arguments, or arguments to the static-factory method if you are using that instead of
a normal constructor. These dependencies are provided to the bean, when the bean is
actually created.

Each property or constructor argument is an actual definition of the value to set, or
a reference to another bean in the container.

Each property or constructor argument which is a value is converted from its specified
format to the actual type of that property or constructor argument. By default Spring
can convert a value supplied in string format to all built-in types, such as int,
long, String, boolean, etc.

The Spring container validates the configuration of each bean as the container is created.
However, the bean properties themselves are not set until the bean is actually created.
Beans that are singleton-scoped and set to be pre-instantiated (the default) are created
when the container is created. Scopes are defined in Section 7.5, “Bean scopes”. Otherwise,
the bean is created only when it is requested. Creation of a bean potentially causes a
graph of beans to be created, as the bean’s dependencies and its dependencies'
dependencies (and so on) are created and assigned. Note that resolution mismatches among
those dependencies may show up late, i.e. on first creation of the affected bean.

Circular dependencies

If you use predominantly constructor injection, it is possible to create an unresolvable
circular dependency scenario.

For example: Class A requires an instance of class B through constructor injection, and
class B requires an instance of class A through constructor injection. If you configure
beans for classes A and B to be injected into each other, the Spring IoC container
detects this circular reference at runtime, and throws a
BeanCurrentlyInCreationException.

One possible solution is to edit the source code of some classes to be configured by
setters rather than constructors. Alternatively, avoid constructor injection and use
setter injection only. In other words, although it is not recommended, you can configure
circular dependencies with setter injection.

Unlike the typical case (with no circular dependencies), a circular dependency
between bean A and bean B forces one of the beans to be injected into the other prior to
being fully initialized itself (a classic chicken/egg scenario).

You can generally trust Spring to do the right thing. It detects configuration problems,
such as references to non-existent beans and circular dependencies, at container
load-time. Spring sets properties and resolves dependencies as late as possible, when
the bean is actually created. This means that a Spring container which has loaded
correctly can later generate an exception when you request an object if there is a
problem creating that object or one of its dependencies. For example, the bean throws an
exception as a result of a missing or invalid property. This potentially delayed
visibility of some configuration issues is why ApplicationContext implementations by
default pre-instantiate singleton beans. At the cost of some upfront time and memory to
create these beans before they are actually needed, you discover configuration issues
when the ApplicationContext is created, not later. You can still override this default
behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.

If no circular dependencies exist, when one or more collaborating beans are being
injected into a dependent bean, each collaborating bean is totally configured prior
to being injected into the dependent bean. This means that if bean A has a dependency on
bean B, the Spring IoC container completely configures bean B prior to invoking the
setter method on bean A. In other words, the bean is instantiated (if not a
pre-instantiated singleton), its dependencies are set, and the relevant lifecycle
methods (such as a configured init method
or the InitializingBean callback method)
are invoked.

Examples of dependency injection

The following example uses XML-based configuration metadata for setter-based DI. A small
part of a Spring XML configuration file specifies some bean definitions:

publicclass ExampleBean {
// a private constructorprivate ExampleBean(...) {
...
}
// a static factory method; the arguments to this method can be// considered the dependencies of the bean that is returned,// regardless of how those arguments are actually used.publicstatic ExampleBean createInstance (
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
ExampleBean eb = new ExampleBean (...);
// some other operations...return eb;
}
}

Arguments to the static factory method are supplied via <constructor-arg/> elements,
exactly the same as if a constructor had actually been used. The type of the class being
returned by the factory method does not have to be of the same type as the class that
contains the static factory method, although in this example it is. An instance
(non-static) factory method would be used in an essentially identical fashion (aside
from the use of the factory-bean attribute instead of the class attribute), so
details will not be discussed here.

7.4.2 Dependencies and configuration in detail

As mentioned in the previous section, you can define bean properties and constructor
arguments as references to other managed beans (collaborators), or as values defined
inline. Spring’s XML-based configuration metadata supports sub-element types within its
<property/> and <constructor-arg/> elements for this purpose.

Straight values (primitives, Strings, and so on)

The value attribute of the <property/> element specifies a property or constructor
argument as a human-readable string representation. Spring’s
conversion service is used to convert these
values from a String to the actual type of the property or argument.

The preceding XML is more succinct; however, typos are discovered at runtime rather than
design time, unless you use an IDE such as IntelliJ
IDEA or the Spring Tool Suite (STS)
that support automatic property completion when you create bean definitions. Such IDE
assistance is highly recommended.

The Spring container converts the text inside the <value/> element into a
java.util.Properties instance by using the JavaBeans PropertyEditor mechanism. This
is a nice shortcut, and is one of a few places where the Spring team do favor the use of
the nested <value/> element over the value attribute style.

The idref element

The idref element is simply an error-proof way to pass the id (string value - not
a reference) of another bean in the container to a <constructor-arg/> or <property/>
element.

The first form is preferable to the second, because using the idref tag allows the
container to validate at deployment time that the referenced, named bean actually
exists. In the second variation, no validation is performed on the value that is passed
to the targetName property of the client bean. Typos are only discovered (with most
likely fatal results) when the client bean is actually instantiated. If the client
bean is a prototype bean, this typo and the resulting exception
may only be discovered long after the container is deployed.

Note

The local attribute on the idref element is no longer supported in the 4.0 beans xsd
since it does not provide value over a regular bean reference anymore. Simply change
your existing idref local references to idref bean when upgrading to the 4.0 schema.

A common place (at least in versions earlier than Spring 2.0) where the <idref/> element
brings value is in the configuration of AOP interceptors in a
ProxyFactoryBean bean definition. Using <idref/> elements when you specify the
interceptor names prevents you from misspelling an interceptor id.

References to other beans (collaborators)

The ref element is the final element inside a <constructor-arg/> or <property/>
definition element. Here you set the value of the specified property of a bean to be a
reference to another bean (a collaborator) managed by the container. The referenced bean
is a dependency of the bean whose property will be set, and it is initialized on demand
as needed before the property is set. (If the collaborator is a singleton bean, it may
be initialized already by the container.) All references are ultimately a reference to
another object. Scoping and validation depend on whether you specify the id/name of the
other object through the bean, local, or parent attributes.

Specifying the target bean through the bean attribute of the <ref/> tag is the most
general form, and allows creation of a reference to any bean in the same container or
parent container, regardless of whether it is in the same XML file. The value of the
bean attribute may be the same as the id attribute of the target bean, or as one of
the values in the name attribute of the target bean.

<refbean="someBean"/>

Specifying the target bean through the parent attribute creates a reference to a bean
that is in a parent container of the current container. The value of the parent
attribute may be the same as either the id attribute of the target bean, or one of the
values in the name attribute of the target bean, and the target bean must be in a
parent container of the current one. You use this bean reference variant mainly when you
have a hierarchy of containers and you want to wrap an existing bean in a parent
container with a proxy that will have the same name as the parent bean.

<!-- in the parent context --><beanid="accountService"class="com.foo.SimpleAccountService"><!-- insert dependencies as required as here --></bean>

<!-- in the child (descendant) context --><beanid="accountService"<!--beannameisthesameastheparentbean-->class="org.springframework.aop.framework.ProxyFactoryBean"><propertyname="target"><refparent="accountService"/><!-- notice how we refer to the parent bean --></property><!-- insert other configuration and dependencies as required here --></bean>

Note

The local attribute on the ref element is no longer supported in the 4.0 beans xsd
since it does not provide value over a regular bean reference anymore. Simply change
your existing ref local references to ref bean when upgrading to the 4.0 schema.

Inner beans

<beanid="outer"class="..."><!-- instead of using a reference to a target bean, simply define the target bean inline --><propertyname="target"><beanclass="com.example.Person"><!-- this is the inner bean --><propertyname="name"value="Fiona Apple"/><propertyname="age"value="25"/></bean></property></bean>

An inner bean definition does not require a defined id or name; if specified, the container
does not use such a value as an identifier. The container also ignores the scope flag on
creation: Inner beans are always anonymous and they are always created with the outer
bean. It is not possible to inject inner beans into collaborating beans other than into
the enclosing bean or to access them independently.

As a corner case, it is possible to receive destruction callbacks from a custom scope, e.g.
for a request-scoped inner bean contained within a singleton bean: The creation of the inner
bean instance will be tied to its containing bean, but destruction callbacks allow it to
participate in the request scope’s lifecycle. This is not a common scenario; inner beans
typically simply share their containing bean’s scope.

Collections

In the <list/>, <set/>, <map/>, and <props/> elements, you set the properties
and arguments of the Java Collection types List, Set, Map, and Properties,
respectively.

<beanid="moreComplexObject"class="example.ComplexObject"><!-- results in a setAdminEmails(java.util.Properties) call --><propertyname="adminEmails"><props><propkey="administrator">[email protected]</prop><propkey="support">[email protected]</prop><propkey="development">[email protected]</prop></props></property><!-- results in a setSomeList(java.util.List) call --><propertyname="someList"><list><value>a list element followed by a reference</value><refbean="myDataSource" /></list></property><!-- results in a setSomeMap(java.util.Map) call --><propertyname="someMap"><map><entrykey="an entry"value="just some string"/><entrykey ="a ref"value-ref="myDataSource"/></map></property><!-- results in a setSomeSet(java.util.Set) call --><propertyname="someSet"><set><value>just some string</value><refbean="myDataSource" /></set></property></bean>

The value of a map key or value, or a set value, can also again be any of the
following elements:

bean | ref | idref | list | set | map | props | value | null

Collection merging

The Spring container also supports the merging of collections. An application
developer can define a parent-style <list/>, <map/>, <set/> or <props/> element,
and have child-style <list/>, <map/>, <set/> or <props/> elements inherit and
override values from the parent collection. That is, the child collection’s values are
the result of merging the elements of the parent and child collections, with the child’s
collection elements overriding values specified in the parent collection.

This section on merging discusses the parent-child bean mechanism. Readers unfamiliar
with parent and child bean definitions may wish to read the
relevant section before continuing.

Notice the use of the merge=true attribute on the <props/> element of the
adminEmails property of the child bean definition. When the child bean is resolved
and instantiated by the container, the resulting instance has an adminEmailsProperties collection that contains the result of the merging of the child’s
adminEmails collection with the parent’s adminEmails collection.

The child Properties collection’s value set inherits all property elements from the
parent <props/>, and the child’s value for the support value overrides the value in
the parent collection.

This merging behavior applies similarly to the <list/>, <map/>, and <set/>
collection types. In the specific case of the <list/> element, the semantics
associated with the List collection type, that is, the notion of an ordered
collection of values, is maintained; the parent’s values precede all of the child list’s
values. In the case of the Map, Set, and Properties collection types, no ordering
exists. Hence no ordering semantics are in effect for the collection types that underlie
the associated Map, Set, and Properties implementation types that the container
uses internally.

Limitations of collection merging

You cannot merge different collection types (such as a Map and a List), and if you
do attempt to do so an appropriate Exception is thrown. The merge attribute must be
specified on the lower, inherited, child definition; specifying the merge attribute on
a parent collection definition is redundant and will not result in the desired merging.

Strongly-typed collection

With the introduction of generic types in Java 5, you can use strongly typed collections.
That is, it is possible to declare a Collection type such that it can only contain
String elements (for example). If you are using Spring to dependency-inject a
strongly-typed Collection into a bean, you can take advantage of Spring’s
type-conversion support such that the elements of your strongly-typed Collection
instances are converted to the appropriate type prior to being added to the Collection.

When the accounts property of the foo bean is prepared for injection, the generics
information about the element type of the strongly-typed Map<String, Float> is
available by reflection. Thus Spring’s type conversion infrastructure recognizes the
various value elements as being of type Float, and the string values 9.99, 2.75, and
3.99 are converted into an actual Float type.

Null and empty string values

Spring treats empty arguments for properties and the like as empty Strings. The
following XML-based configuration metadata snippet sets the email property to the empty
String value ("").

XML shortcut with the p-namespace

The p-namespace enables you to use the bean element’s attributes, instead of nested
<property/> elements, to describe your property values and/or collaborating beans.

Spring supports extensible configuration formats with namespaces, which are
based on an XML Schema definition. The beans configuration format discussed in this
chapter is defined in an XML Schema document. However, the p-namespace is not defined in
an XSD file and exists only in the core of Spring.

The following example shows two XML snippets that resolve to the same result: The first
uses standard XML format and the second uses the p-namespace.

The example shows an attribute in the p-namespace called email in the bean definition.
This tells Spring to include a property declaration. As previously mentioned, the
p-namespace does not have a schema definition, so you can set the name of the attribute
to the property name.

This next example includes two more bean definitions that both have a reference to
another bean:

As you can see, this example includes not only a property value using the p-namespace,
but also uses a special format to declare property references. Whereas the first bean
definition uses <property name="spouse" ref="jane"/> to create a reference from bean
john to bean jane, the second bean definition uses p:spouse-ref="jane" as an
attribute to do the exact same thing. In this case spouse is the property name,
whereas the -ref part indicates that this is not a straight value but rather a
reference to another bean.

Note

The p-namespace is not as flexible as the standard XML format. For example, the format
for declaring property references clashes with properties that end in Ref, whereas the
standard XML format does not. We recommend that you choose your approach carefully and
communicate this to your team members, to avoid producing XML documents that use all
three approaches at the same time.

The c: namespace uses the same conventions as the p: one (trailing -ref for bean
references) for setting the constructor arguments by their names. And just as well, it
needs to be declared even though it is not defined in an XSD schema (but it exists
inside the Spring core).

For the rare cases where the constructor argument names are not available (usually if
the bytecode was compiled without debugging information), one can use fallback to the
argument indexes:

The foo bean has a fred property, which has a bob property, which has a sammy
property, and that final sammy property is being set to the value 123. In order for
this to work, the fred property of foo, and the bob property of fred must not be
null after the bean is constructed, or a NullPointerException is thrown.

7.4.3 Using depends-on

If a bean is a dependency of another that usually means that one bean is set as a
property of another. Typically you accomplish this with the <ref/>
element in XML-based configuration metadata. However, sometimes dependencies between
beans are less direct; for example, a static initializer in a class needs to be
triggered, such as database driver registration. The depends-on attribute can
explicitly force one or more beans to be initialized before the bean using this element
is initialized. The following example uses the depends-on attribute to express a
dependency on a single bean:

The depends-on attribute in the bean definition can specify both an initialization
time dependency and, in the case of singleton beans
only, a corresponding destroy time dependency. Dependent beans that define a
depends-on relationship with a given bean are destroyed first, prior to the given bean
itself being destroyed. Thus depends-on can also control shutdown order.

7.4.4 Lazy-initialized beans

By default, ApplicationContext implementations eagerly create and configure all
singleton beans as part of the initialization
process. Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately, as opposed to hours
or even days later. When this behavior is not desirable, you can prevent
pre-instantiation of a singleton bean by marking the bean definition as
lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean
instance when it is first requested, rather than at startup.

In XML, this behavior is controlled by the lazy-init attribute on the <bean/>
element; for example:

When the preceding configuration is consumed by an ApplicationContext, the bean named
lazy is not eagerly pre-instantiated when the ApplicationContext is starting up,
whereas the not.lazy bean is eagerly pre-instantiated.

However, when a lazy-initialized bean is a dependency of a singleton bean that is
not lazy-initialized, the ApplicationContext creates the lazy-initialized bean at
startup, because it must satisfy the singleton’s dependencies. The lazy-initialized bean
is injected into a singleton bean elsewhere that is not lazy-initialized.

You can also control lazy-initialization at the container level by using the
default-lazy-init attribute on the <beans/> element; for example:

<beansdefault-lazy-init="true"><!-- no beans will be pre-instantiated... --></beans>

7.4.5 Autowiring collaborators

The Spring container can autowire relationships between collaborating beans. You can
allow Spring to resolve collaborators (other beans) automatically for your bean by
inspecting the contents of the ApplicationContext. Autowiring has the following
advantages:

Autowiring can significantly reduce the need to specify properties or constructor
arguments. (Other mechanisms such as a bean template
discussed elsewhere in this chapter are also valuable
in this regard.)

Autowiring can update a configuration as your objects evolve. For example, if you need
to add a dependency to a class, that dependency can be satisfied automatically without
you needing to modify the configuration. Thus autowiring can be especially useful
during development, without negating the option of switching to explicit wiring when
the code base becomes more stable.

When using XML-based configuration metadata [2], you specify autowire
mode for a bean definition with the autowire attribute of the <bean/> element. The
autowiring functionality has four modes. You specify autowiring per bean and thus
can choose which ones to autowire.

Table 7.2. Autowiring modes

Mode

Explanation

no

(Default) No autowiring. Bean references must be defined via a ref element. Changing
the default setting is not recommended for larger deployments, because specifying
collaborators explicitly gives greater control and clarity. To some extent, it
documents the structure of a system.

byName

Autowiring by property name. Spring looks for a bean with the same name as the
property that needs to be autowired. For example, if a bean definition is set to
autowire by name, and it contains a master property (that is, it has a
setMaster(..) method), Spring looks for a bean definition named master, and uses
it to set the property.

byType

Allows a property to be autowired if exactly one bean of the property type exists in
the container. If more than one exists, a fatal exception is thrown, which indicates
that you may not use byType autowiring for that bean. If there are no matching
beans, nothing happens; the property is not set.

constructor

Analogous to byType, but applies to constructor arguments. If there is not exactly
one bean of the constructor argument type in the container, a fatal error is raised.

With byType or constructor autowiring mode, you can wire arrays and
typed-collections. In such cases all autowire candidates within the container that
match the expected type are provided to satisfy the dependency. You can autowire
strongly-typed Maps if the expected key type is String. An autowired Maps values will
consist of all bean instances that match the expected type, and the Maps keys will
contain the corresponding bean names.

You can combine autowire behavior with dependency checking, which is performed after
autowiring completes.

Limitations and disadvantages of autowiring

Autowiring works best when it is used consistently across a project. If autowiring is
not used in general, it might be confusing to developers to use it to wire only one or
two bean definitions.

Consider the limitations and disadvantages of autowiring:

Explicit dependencies in property and constructor-arg settings always override
autowiring. You cannot autowire so-called simple properties such as primitives,
Strings, and Classes (and arrays of such simple properties). This limitation is
by-design.

Autowiring is less exact than explicit wiring. Although, as noted in the above table,
Spring is careful to avoid guessing in case of ambiguity that might have unexpected
results, the relationships between your Spring-managed objects are no longer
documented explicitly.

Wiring information may not be available to tools that may generate documentation from
a Spring container.

Multiple bean definitions within the container may match the type specified by the
setter method or constructor argument to be autowired. For arrays, collections, or
Maps, this is not necessarily a problem. However for dependencies that expect a single
value, this ambiguity is not arbitrarily resolved. If no unique bean definition is
available, an exception is thrown.

In the latter scenario, you have several options:

Abandon autowiring in favor of explicit wiring.

Avoid autowiring for a bean definition by setting its autowire-candidate attributes
to false as described in the next section.

Designate a single bean definition as the primary candidate by setting the
primary attribute of its <bean/> element to true.

Excluding a bean from autowiring

On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set
the autowire-candidate attribute of the <bean/> element to false; the container
makes that specific bean definition unavailable to the autowiring infrastructure
(including annotation style configurations such as @Autowired).

Note

The autowire-candidate attribute is designed to only affect type-based autowiring.
It does not affect explicit references by name, which will get resolved even if the
specified bean is not marked as an autowire candidate. As a consequence, autowiring
by name will nevertheless inject a bean if the name matches.

You can also limit autowire candidates based on pattern-matching against bean names. The
top-level <beans/> element accepts one or more patterns within its
default-autowire-candidates attribute. For example, to limit autowire candidate status
to any bean whose name ends with Repository, provide a value of *Repository. To
provide multiple patterns, define them in a comma-separated list. An explicit value of
true or false for a bean definitions autowire-candidate attribute always takes
precedence, and for such beans, the pattern matching rules do not apply.

These techniques are useful for beans that you never want to be injected into other
beans by autowiring. It does not mean that an excluded bean cannot itself be configured
using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.

7.4.6 Method injection

In most application scenarios, most beans in the container are
singletons. When a singleton bean needs to
collaborate with another singleton bean, or a non-singleton bean needs to collaborate
with another non-singleton bean, you typically handle the dependency by defining one
bean as a property of the other. A problem arises when the bean lifecycles are
different. Suppose singleton bean A needs to use non-singleton (prototype) bean B,
perhaps on each method invocation on A. The container only creates the singleton bean A
once, and thus only gets one opportunity to set the properties. The container cannot
provide bean A with a new instance of bean B every time one is needed.

The preceding is not desirable, because the business code is aware of and coupled to the
Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC
container, allows this use case to be handled in a clean fashion.

You can read more about the motivation for Method Injection in
this blog entry.

Lookup method injection

Lookup method injection is the ability of the container to override methods on
container managed beans, to return the lookup result for another named bean in the
container. The lookup typically involves a prototype bean as in the scenario described
in the preceding section. The Spring Framework implements this method injection by using
bytecode generation from the CGLIB library to generate dynamically a subclass that
overrides the method.

Note

For this dynamic subclassing to work, the class that the Spring bean container will
subclass cannot be final, and the method to be overridden cannot be final either.

Unit-testing a class that has an abstract method requires you to subclass the class
yourself and to supply a stub implementation of the abstract method.

Concrete methods are also necessary for component scanning which requires concrete
classes to pick up.

A further key limitation is that lookup methods won’t work with factory methods and
in particular not with @Bean methods in configuration classes, since the container
is not in charge of creating the instance in that case and therefore cannot create
a runtime-generated subclass on the fly.

Looking at the CommandManager class in the previous code snippet, you see that the
Spring container will dynamically override the implementation of the createCommand()
method. Your CommandManager class will not have any Spring dependencies, as can be
seen in the reworked example:

package fiona.apple;
// no more Spring imports!publicabstractclass CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?protectedabstract Command createCommand();
}

In the client class containing the method to be injected (the CommandManager in this
case), the method to be injected requires a signature of the following form:

If the method is abstract, the dynamically-generated subclass implements the method.
Otherwise, the dynamically-generated subclass overrides the concrete method defined in
the original class. For example:

The bean identified as commandManager calls its own method createCommand()
whenever it needs a new instance of the myCommand bean. You must be careful to deploy
the myCommand bean as a prototype, if that is actually what is needed. If it is
as a singleton, the same instance of the myCommand
bean is returned each time.

Alternatively, within the annotation-based component model, you may declare a lookup
method through the @Lookup annotation:

Note that you will typically declare such annotated lookup methods with a concrete
stub implementation, in order for them to be compatible with Spring’s component
scanning rules where abstract classes get ignored by default. This limitation does not
apply in case of explicitly registered or explicitly imported bean classes.

The interested reader may also find the ServiceLocatorFactoryBean (in the
org.springframework.beans.factory.config package) to be of use.

Arbitrary method replacement

A less useful form of method injection than lookup method injection is the ability to
replace arbitrary methods in a managed bean with another method implementation. Users
may safely skip the rest of this section until the functionality is actually needed.

With XML-based configuration metadata, you can use the replaced-method element to
replace an existing method implementation with another, for a deployed bean. Consider
the following class, with a method computeValue, which we want to override:

You can use one or more contained <arg-type/> elements within the <replaced-method/>
element to indicate the method signature of the method being overridden. The signature
for the arguments is necessary only if the method is overloaded and multiple variants
exist within the class. For convenience, the type string for an argument may be a
substring of the fully qualified type name. For example, the following all match
java.lang.String:

java.lang.String
String
Str

Because the number of arguments is often enough to distinguish between each possible
choice, this shortcut can save a lot of typing, by allowing you to type only the
shortest string that will match an argument type.

7.5 Bean scopes

When you create a bean definition, you create a recipe for creating actual instances
of the class defined by that bean definition. The idea that a bean definition is a
recipe is important, because it means that, as with a class, you can create many object
instances from a single recipe.

You can control not only the various dependencies and configuration values that are to
be plugged into an object that is created from a particular bean definition, but also
the scope of the objects created from a particular bean definition. This approach is
powerful and flexible in that you can choose the scope of the objects you create
through configuration instead of having to bake in the scope of an object at the Java
class level. Beans can be defined to be deployed in one of a number of scopes: out of
the box, the Spring Framework supports seven scopes, five of which are available only if
you use a web-aware ApplicationContext.

The following scopes are supported out of the box. You can also create
a custom scope.

Scopes a single bean definition to the lifecycle of a single HTTP request; that is,
each HTTP request has its own instance of a bean created off the back of a single bean
definition. Only valid in the context of a web-aware Spring ApplicationContext.

7.5.1 The singleton scope

Only one shared instance of a singleton bean is managed, and all requests for beans
with an id or ids matching that bean definition result in that one specific bean
instance being returned by the Spring container.

To put it another way, when you define a bean definition and it is scoped as a
singleton, the Spring IoC container creates exactly one instance of the object
defined by that bean definition. This single instance is stored in a cache of such
singleton beans, and all subsequent requests and references for that named bean
return the cached object.

Spring’s concept of a singleton bean differs from the Singleton pattern as defined in
the Gang of Four (GoF) patterns book. The GoF Singleton hard-codes the scope of an
object such that one and only one instance of a particular class is created per
ClassLoader. The scope of the Spring singleton is best described as per container
and per bean. This means that if you define one bean for a particular class in a
single Spring container, then the Spring container creates one and only one instance
of the class defined by that bean definition. The singleton scope is the default scope
in Spring. To define a bean as a singleton in XML, you would write, for example:

<beanid="accountService"class="com.foo.DefaultAccountService"/><!-- the following is equivalent, though redundant (singleton scope is the default) --><beanid="accountService"class="com.foo.DefaultAccountService"scope="singleton"/>

7.5.2 The prototype scope

The non-singleton, prototype scope of bean deployment results in the creation of a new
bean instance every time a request for that specific bean is made. That is, the bean
is injected into another bean or you request it through a getBean() method call on the
container. As a rule, use the prototype scope for all stateful beans and the singleton
scope for stateless beans.

The following diagram illustrates the Spring prototype scope. A data access object
(DAO) is not typically configured as a prototype, because a typical DAO does not hold
any conversational state; it was just easier for this author to reuse the core of the
singleton diagram.

In contrast to the other scopes, Spring does not manage the complete lifecycle of a
prototype bean: the container instantiates, configures, and otherwise assembles a
prototype object, and hands it to the client, with no further record of that prototype
instance. Thus, although initialization lifecycle callback methods are called on all
objects regardless of scope, in the case of prototypes, configured destruction
lifecycle callbacks are not called. The client code must clean up prototype-scoped
objects and release expensive resources that the prototype bean(s) are holding. To get
the Spring container to release resources held by prototype-scoped beans, try using a
custom bean post-processor, which holds a reference to
beans that need to be cleaned up.

In some respects, the Spring container’s role in regard to a prototype-scoped bean is a
replacement for the Java new operator. All lifecycle management past that point must
be handled by the client. (For details on the lifecycle of a bean in the Spring
container, see Section 7.6.1, “Lifecycle callbacks”.)

7.5.3 Singleton beans with prototype-bean dependencies

When you use singleton-scoped beans with dependencies on prototype beans, be aware that
dependencies are resolved at instantiation time. Thus if you dependency-inject a
prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated
and then dependency-injected into the singleton bean. The prototype instance is the sole
instance that is ever supplied to the singleton-scoped bean.

However, suppose you want the singleton-scoped bean to acquire a new instance of the
prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a
prototype-scoped bean into your singleton bean, because that injection occurs only
once, when the Spring container is instantiating the singleton bean and resolving
and injecting its dependencies. If you need a new instance of a prototype bean at
runtime more than once, see Section 7.4.6, “Method injection”

The request, session, globalSession, application, and websocket scopes are
only available if you use a web-aware Spring ApplicationContext implementation
(such as XmlWebApplicationContext). If you use these scopes with regular Spring IoC
containers such as the ClassPathXmlApplicationContext, an IllegalStateException will
be thrown complaining about an unknown bean scope.

Initial web configuration

To support the scoping of beans at the request, session, globalSession,
application, and websocket levels (web-scoped beans), some minor initial
configuration is required before you define your beans. (This initial setup is not
required for the standard scopes, singleton and prototype.)

How you accomplish this initial setup depends on your particular Servlet environment.

If you access scoped beans within Spring Web MVC, in effect, within a request that is
processed by the Spring DispatcherServlet or DispatcherPortlet, then no special
setup is necessary: DispatcherServlet and DispatcherPortlet already expose all
relevant state.

If you use a Servlet 2.5 web container, with requests processed outside of Spring’s
DispatcherServlet (for example, when using JSF or Struts), you need to register the
org.springframework.web.context.request.RequestContextListenerServletRequestListener.
For Servlet 3.0+, this can be done programmatically via the WebApplicationInitializer
interface. Alternatively, or for older containers, add the following declaration to
your web application’s web.xml file:

Alternatively, if there are issues with your listener setup, consider using Spring’s
RequestContextFilter. The filter mapping depends on the surrounding web
application configuration, so you have to change it as appropriate.

DispatcherServlet, RequestContextListener, and RequestContextFilter all do exactly
the same thing, namely bind the HTTP request object to the Thread that is servicing
that request. This makes beans that are request- and session-scoped available further
down the call chain.

Request scope

Consider the following XML configuration for a bean definition:

<beanid="loginAction"class="com.foo.LoginAction"scope="request"/>

The Spring container creates a new instance of the LoginAction bean by using the
loginAction bean definition for each and every HTTP request. That is, the
loginAction bean is scoped at the HTTP request level. You can change the internal
state of the instance that is created as much as you want, because other instances
created from the same loginAction bean definition will not see these changes in state;
they are particular to an individual request. When the request completes processing, the
bean that is scoped to the request is discarded.

When using annotation-driven components or Java Config, the @RequestScope annotation
can be used to assign a component to the request scope.

@RequestScope@Componentpublicclass LoginAction {
// ...
}

Session scope

The Spring container creates a new instance of the UserPreferences bean by using the
userPreferences bean definition for the lifetime of a single HTTP Session. In other
words, the userPreferences bean is effectively scoped at the HTTP Session level. As
with request-scoped beans, you can change the internal state of the instance that is
created as much as you want, knowing that other HTTP Session instances that are also
using instances created from the same userPreferences bean definition do not see these
changes in state, because they are particular to an individual HTTP Session. When the
HTTP Session is eventually discarded, the bean that is scoped to that particular HTTP
Session is also discarded.

When using annotation-driven components or Java Config, the @SessionScope annotation
can be used to assign a component to the session scope.

@SessionScope@Componentpublicclass UserPreferences {
// ...
}

Global session scope

The globalSession scope is similar to the standard HTTP Session scope
(described above), and applies only in the context of
portlet-based web applications. The portlet specification defines the notion of a global
Session that is shared among all portlets that make up a single portlet web
application. Beans defined at the globalSession scope are scoped (or bound) to the
lifetime of the global portlet Session.

If you write a standard Servlet-based web application and you define one or more beans
as having globalSession scope, the standard HTTP Session scope is used, and no
error is raised.

Application scope

The Spring container creates a new instance of the AppPreferences bean by using the
appPreferences bean definition once for the entire web application. That is, the
appPreferences bean is scoped at the ServletContext level, stored as a regular
ServletContext attribute. This is somewhat similar to a Spring singleton bean but
differs in two important ways: It is a singleton per ServletContext, not per Spring
'ApplicationContext' (for which there may be several in any given web application),
and it is actually exposed and therefore visible as a ServletContext attribute.

When using annotation-driven components or Java Config, the @ApplicationScope
annotation can be used to assign a component to the application scope.

@ApplicationScope@Componentpublicclass AppPreferences {
// ...
}

Scoped beans as dependencies

The Spring IoC container manages not only the instantiation of your objects (beans),
but also the wiring up of collaborators (or dependencies). If you want to inject (for
example) an HTTP request scoped bean into another bean of a longer-lived scope, you may
choose to inject an AOP proxy in place of the scoped bean. That is, you need to inject
a proxy object that exposes the same public interface as the scoped object but that can
also retrieve the real target object from the relevant scope (such as an HTTP request)
and delegate method calls onto the real object.

Note

You may also use <aop:scoped-proxy/> between beans that are scoped as singleton,
with the reference then going through an intermediate proxy that is serializable
and therefore able to re-obtain the target singleton bean on deserialization.

When declaring <aop:scoped-proxy/> against a bean of scope prototype, every method
call on the shared proxy will lead to the creation of a new target instance which the
call is then being forwarded to.

Also, scoped proxies are not the only way to access beans from shorter scopes in a
lifecycle-safe fashion. You may also simply declare your injection point (i.e. the
constructor/setter argument or autowired field) as ObjectFactory<MyTargetBean>,
allowing for a getObject() call to retrieve the current instance on demand every
time it is needed - without holding on to the instance or storing it separately.

The JSR-330 variant of this is called Provider, used with a Provider<MyTargetBean>
declaration and a corresponding get() call for every retrieval attempt.
See here for more details on JSR-330 overall.

The configuration in the following example is only one line, but it is important to
understand the "why" as well as the "how" behind it.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:aop="http://www.springframework.org/schema/aop"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd"><!-- an HTTP Session-scoped bean exposed as a proxy --><beanid="userPreferences"class="com.foo.UserPreferences"scope="session"><!-- instructs the container to proxy the surrounding bean --><aop:scoped-proxy/></bean><!-- a singleton-scoped bean injected with a proxy to the above bean --><beanid="userService"class="com.foo.SimpleUserService"><!-- a reference to the proxied userPreferences bean --><propertyname="userPreferences"ref="userPreferences"/></bean></beans>

To create such a proxy, you insert a child <aop:scoped-proxy/> element into a scoped
bean definition (see the section called “Choosing the type of proxy to create” and
Chapter 41, XML Schema-based configuration). Why do definitions of beans scoped at the request, session,
globalSession and custom-scope levels require the <aop:scoped-proxy/> element?
Let’s examine the following singleton bean definition and contrast it with what you need
to define for the aforementioned scopes (note that the following userPreferences bean
definition as it stands is incomplete).

In the preceding example, the singleton bean userManager is injected with a reference
to the HTTP Session-scoped bean userPreferences. The salient point here is that the
userManager bean is a singleton: it will be instantiated exactly once per
container, and its dependencies (in this case only one, the userPreferences bean) are
also injected only once. This means that the userManager bean will only operate on the
exact same userPreferences object, that is, the one that it was originally injected
with.

This is not the behavior you want when injecting a shorter-lived scoped bean into a
longer-lived scoped bean, for example injecting an HTTP Session-scoped collaborating
bean as a dependency into singleton bean. Rather, you need a single userManager
object, and for the lifetime of an HTTP Session, you need a userPreferences object
that is specific to said HTTP Session. Thus the container creates an object that
exposes the exact same public interface as the UserPreferences class (ideally an
object that is aUserPreferences instance) which can fetch the real
UserPreferences object from the scoping mechanism (HTTP request, Session, etc.). The
container injects this proxy object into the userManager bean, which is unaware that
this UserPreferences reference is a proxy. In this example, when a UserManager
instance invokes a method on the dependency-injected UserPreferences object, it
actually is invoking a method on the proxy. The proxy then fetches the real
UserPreferences object from (in this case) the HTTP Session, and delegates the
method invocation onto the retrieved real UserPreferences object.

Thus you need the following, correct and complete, configuration when injecting
request-, session-, and globalSession-scoped beans into collaborating objects:

Choosing the type of proxy to create

By default, when the Spring container creates a proxy for a bean that is marked up with
the <aop:scoped-proxy/> element, a CGLIB-based class proxy is created.

Note

CGLIB proxies only intercept public method calls! Do not call non-public methods
on such a proxy; they will not be delegated to the actual scoped target object.

Alternatively, you can configure the Spring container to create standard JDK
interface-based proxies for such scoped beans, by specifying false for the value of
the proxy-target-class attribute of the <aop:scoped-proxy/> element. Using JDK
interface-based proxies means that you do not need additional libraries in your
application classpath to effect such proxying. However, it also means that the class of
the scoped bean must implement at least one interface, and that all collaborators
into which the scoped bean is injected must reference the bean through one of its
interfaces.

7.5.5 Custom scopes

The bean scoping mechanism is extensible; You can define your own
scopes, or even redefine existing scopes, although the latter is considered bad practice
and you cannot override the built-in singleton and prototype scopes.

Creating a custom scope

To integrate your custom scope(s) into the Spring container, you need to implement the
org.springframework.beans.factory.config.Scope interface, which is described in this
section. For an idea of how to implement your own scopes, see the Scope
implementations that are supplied with the Spring Framework itself and the
Scope javadocs,
which explains the methods you need to implement in more detail.

The Scope interface has four methods to get objects from the scope, remove them from
the scope, and allow them to be destroyed.

The following method returns the object from the underlying scope. The session scope
implementation, for example, returns the session-scoped bean (and if it does not exist,
the method returns a new instance of the bean, after having bound it to the session for
future reference).

Object get(String name, ObjectFactory objectFactory)

The following method removes the object from the underlying scope. The session scope
implementation for example, removes the session-scoped bean from the underlying session.
The object should be returned, but you can return null if the object with the specified
name is not found.

Object remove(String name)

The following method registers the callbacks the scope should execute when it is
destroyed or when the specified object in the scope is destroyed. Refer to the javadocs
or a Spring scope implementation for more information on destruction callbacks.

The following method obtains the conversation identifier for the underlying scope. This
identifier is different for each scope. For a session scoped implementation, this
identifier can be the session identifier.

String getConversationId()

Using a custom scope

After you write and test one or more custom Scope implementations, you need to make
the Spring container aware of your new scope(s). The following method is the central
method to register a new Scope with the Spring container:

void registerScope(String scopeName, Scope scope);

This method is declared on the ConfigurableBeanFactory interface, which is available
on most of the concrete ApplicationContext implementations that ship with Spring via
the BeanFactory property.

The first argument to the registerScope(..) method is the unique name associated with
a scope; examples of such names in the Spring container itself are singleton and
prototype. The second argument to the registerScope(..) method is an actual instance
of the custom Scope implementation that you wish to register and use.

Suppose that you write your custom Scope implementation, and then register it as below.

Note

The example below uses SimpleThreadScope which is included with Spring, but not
registered by default. The instructions would be the same for your own custom Scope
implementations.

When you place <aop:scoped-proxy/> in a FactoryBean implementation, it is the factory
bean itself that is scoped, not the object returned from getObject().

7.6 Customizing the nature of a bean

7.6.1 Lifecycle callbacks

To interact with the container’s management of the bean lifecycle, you can implement the
Spring InitializingBean and DisposableBean interfaces. The container calls
afterPropertiesSet() for the former and destroy() for the latter to allow the bean
to perform certain actions upon initialization and destruction of your beans.

Tip

The JSR-250 @PostConstruct and @PreDestroy annotations are generally considered best
practice for receiving lifecycle callbacks in a modern Spring application. Using these
annotations means that your beans are not coupled to Spring specific interfaces. For
details see Section 7.9.8, “@PostConstruct and @PreDestroy”.

If you don’t want to use the JSR-250 annotations but you are still looking to remove
coupling consider the use of init-method and destroy-method object definition metadata.

Internally, the Spring Framework uses BeanPostProcessor implementations to process any
callback interfaces it can find and call the appropriate methods. If you need custom
features or other lifecycle behavior Spring does not offer out-of-the-box, you can
implement a BeanPostProcessor yourself. For more information, see
Section 7.8, “Container Extension Points”.

In addition to the initialization and destruction callbacks, Spring-managed objects may
also implement the Lifecycle interface so that those objects can participate in the
startup and shutdown process as driven by the container’s own lifecycle.

The lifecycle callback interfaces are described in this section.

Initialization callbacks

The org.springframework.beans.factory.InitializingBean interface allows a bean to
perform initialization work after all necessary properties on the bean have been set by
the container. The InitializingBean interface specifies a single method:

void afterPropertiesSet() throws Exception;

It is recommended that you do not use the InitializingBean interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PostConstruct annotation or
specify a POJO initialization method. In the case of XML-based configuration metadata,
you use the init-method attribute to specify the name of the method that has a void
no-argument signature. With Java config, you use the initMethod attribute of @Bean,
see the section called “Receiving lifecycle callbacks”. For example, the following:

Destruction callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface allows a
bean to get a callback when the container containing it is destroyed. The
DisposableBean interface specifies a single method:

void destroy() throws Exception;

It is recommended that you do not use the DisposableBean callback interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PreDestroy annotation or
specify a generic method that is supported by bean definitions. With XML-based
configuration metadata, you use the destroy-method attribute on the <bean/>.
With Java config, you use the destroyMethod attribute of @Bean, see
the section called “Receiving lifecycle callbacks”. For example, the following definition:

The destroy-method attribute of a <bean> element can be assigned a special
(inferred) value which instructs Spring to automatically detect a public close or
shutdown method on the specific bean class (any class that implements
java.lang.AutoCloseable or java.io.Closeable would therefore match). This special
(inferred) value can also be set on the default-destroy-method attribute of a
<beans> element to apply this behavior to an entire set of beans (see
the section called “Default initialization and destroy methods”). Note that this is the
default behavior with Java config.

Default initialization and destroy methods

When you write initialization and destroy method callbacks that do not use the
Spring-specific InitializingBean and DisposableBean callback interfaces, you
typically write methods with names such as init(), initialize(), dispose(), and so
on. Ideally, the names of such lifecycle callback methods are standardized across a
project so that all developers use the same method names and ensure consistency.

You can configure the Spring container to look for named initialization and destroy
callback method names on every bean. This means that you, as an application
developer, can write your application classes and use an initialization callback called
init(), without having to configure an init-method="init" attribute with each bean
definition. The Spring IoC container calls that method when the bean is created (and in
accordance with the standard lifecycle callback contract described previously). This
feature also enforces a consistent naming convention for initialization and destroy
method callbacks.

Suppose that your initialization callback methods are named init() and destroy
callback methods are named destroy(). Your class will resemble the class in the
following example.

The presence of the default-init-method attribute on the top-level <beans/> element
attribute causes the Spring IoC container to recognize a method called init on beans
as the initialization method callback. When a bean is created and assembled, if the bean
class has such a method, it is invoked at the appropriate time.

You configure destroy method callbacks similarly (in XML, that is) by using the
default-destroy-method attribute on the top-level <beans/> element.

Where existing bean classes already have callback methods that are named at variance
with the convention, you can override the default by specifying (in XML, that is) the
method name using the init-method and destroy-method attributes of the <bean/>
itself.

The Spring container guarantees that a configured initialization callback is called
immediately after a bean is supplied with all dependencies. Thus the initialization
callback is called on the raw bean reference, which means that AOP interceptors and so
forth are not yet applied to the bean. A target bean is fully created first,
then an AOP proxy (for example) with its interceptor chain is applied. If the target
bean and the proxy are defined separately, your code can even interact with the raw
target bean, bypassing the proxy. Hence, it would be inconsistent to apply the
interceptors to the init method, because doing so would couple the lifecycle of the
target bean with its proxy/interceptors and leave strange semantics when your code
interacts directly to the raw target bean.

If multiple lifecycle mechanisms are configured for a bean, and each mechanism is
configured with a different method name, then each configured method is executed in the
order listed below. However, if the same method name is configured - for example,
init() for an initialization method - for more than one of these lifecycle mechanisms,
that method is executed once, as explained in the preceding section.

Multiple lifecycle mechanisms configured for the same bean, with different
initialization methods, are called as follows:

Methods annotated with @PostConstruct

afterPropertiesSet() as defined by the InitializingBean callback interface

A custom configured init() method

Destroy methods are called in the same order:

Methods annotated with @PreDestroy

destroy() as defined by the DisposableBean callback interface

A custom configured destroy() method

Startup and shutdown callbacks

The Lifecycle interface defines the essential methods for any object that has its own
lifecycle requirements (e.g. starts and stops some background process):

Any Spring-managed object may implement that interface. Then, when the
ApplicationContext itself receives start and stop signals, e.g. for a stop/restart
scenario at runtime, it will cascade those calls to all Lifecycle implementations
defined within that context. It does this by delegating to a LifecycleProcessor:

Notice that the LifecycleProcessor is itself an extension of the Lifecycle
interface. It also adds two other methods for reacting to the context being refreshed
and closed.

Tip

Note that the regular org.springframework.context.Lifecycle interface is just a plain
contract for explicit start/stop notifications and does NOT imply auto-startup at context
refresh time. Consider implementing org.springframework.context.SmartLifecycle instead
for fine-grained control over auto-startup of a specific bean (including startup phases).
Also, please note that stop notifications are not guaranteed to come before destruction:
On regular shutdown, all Lifecycle beans will first receive a stop notification before
the general destruction callbacks are being propagated; however, on hot refresh during a
context’s lifetime or on aborted refresh attempts, only destroy methods will be called.

The order of startup and shutdown invocations can be important. If a "depends-on"
relationship exists between any two objects, the dependent side will start after its
dependency, and it will stop before its dependency. However, at times the direct
dependencies are unknown. You may only know that objects of a certain type should start
prior to objects of another type. In those cases, the SmartLifecycle interface defines
another option, namely the getPhase() method as defined on its super-interface,
Phased.

When starting, the objects with the lowest phase start first, and when stopping, the
reverse order is followed. Therefore, an object that implements SmartLifecycle and
whose getPhase() method returns Integer.MIN_VALUE would be among the first to start
and the last to stop. At the other end of the spectrum, a phase value of
Integer.MAX_VALUE would indicate that the object should be started last and stopped
first (likely because it depends on other processes to be running). When considering the
phase value, it’s also important to know that the default phase for any "normal"
Lifecycle object that does not implement SmartLifecycle would be 0. Therefore, any
negative phase value would indicate that an object should start before those standard
components (and stop after them), and vice versa for any positive phase value.

As you can see the stop method defined by SmartLifecycle accepts a callback. Any
implementation must invoke that callback’s run() method after that implementation’s
shutdown process is complete. That enables asynchronous shutdown where necessary since
the default implementation of the LifecycleProcessor interface,
DefaultLifecycleProcessor, will wait up to its timeout value for the group of objects
within each phase to invoke that callback. The default per-phase timeout is 30 seconds.
You can override the default lifecycle processor instance by defining a bean named
"lifecycleProcessor" within the context. If you only want to modify the timeout, then
defining the following would be sufficient:

As mentioned, the LifecycleProcessor interface defines callback methods for the
refreshing and closing of the context as well. The latter will simply drive the shutdown
process as if stop() had been called explicitly, but it will happen when the context is
closing. The 'refresh' callback on the other hand enables another feature of
SmartLifecycle beans. When the context is refreshed (after all objects have been
instantiated and initialized), that callback will be invoked, and at that point the
default lifecycle processor will check the boolean value returned by each
SmartLifecycle object’s isAutoStartup() method. If "true", then that object will be
started at that point rather than waiting for an explicit invocation of the context’s or
its own start() method (unlike the context refresh, the context start does not happen
automatically for a standard context implementation). The "phase" value as well as any
"depends-on" relationships will determine the startup order in the same way as described
above.

Shutting down the Spring IoC container gracefully in non-web applications

Note

This section applies only to non-web applications. Spring’s web-based
ApplicationContext implementations already have code in place to shut down the Spring
IoC container gracefully when the relevant web application is shut down.

If you are using Spring’s IoC container in a non-web application environment; for
example, in a rich client desktop environment; you register a shutdown hook with the
JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your
singleton beans so that all resources are released. Of course, you must still configure
and implement these destroy callbacks correctly.

To register a shutdown hook, you call the registerShutdownHook() method that is
declared on the ConfigurableApplicationContext interface:

7.6.2 ApplicationContextAware and BeanNameAware

When an ApplicationContext creates an object instance that implements the
org.springframework.context.ApplicationContextAware interface, the instance is provided
with a reference to that ApplicationContext.

Thus beans can manipulate programmatically the ApplicationContext that created them,
through the ApplicationContext interface, or by casting the reference to a known
subclass of this interface, such as ConfigurableApplicationContext, which exposes
additional functionality. One use would be the programmatic retrieval of other beans.
Sometimes this capability is useful; however, in general you should avoid it, because it
couples the code to Spring and does not follow the Inversion of Control style, where
collaborators are provided to beans as properties. Other methods of the
ApplicationContext provide access to file resources, publishing application events, and
accessing a MessageSource. These additional features are described in
Section 7.15, “Additional Capabilities of the ApplicationContext”

As of Spring 2.5, autowiring is another alternative to obtain reference to the
ApplicationContext. The "traditional" constructor and byType autowiring modes (as
described in Section 7.4.5, “Autowiring collaborators”) can provide a dependency of type
ApplicationContext for a constructor argument or setter method parameter,
respectively. For more flexibility, including the ability to autowire fields and
multiple parameter methods, use the new annotation-based autowiring features. If you do,
the ApplicationContext is autowired into a field, constructor argument, or method
parameter that is expecting the ApplicationContext type if the field, constructor, or
method in question carries the @Autowired annotation. For more information, see
Section 7.9.2, “@Autowired”.

When an ApplicationContext creates a class that implements the
org.springframework.beans.factory.BeanNameAware interface, the class is provided with
a reference to the name defined in its associated object definition.

The callback is invoked after population of normal bean properties but before an
initialization callback such as InitializingBeanafterPropertiesSet or a custom
init-method.

7.6.3 Other Aware interfaces

Besides ApplicationContextAware and BeanNameAware discussed above, Spring offers a
range of Aware interfaces that allow beans to indicate to the container that they
require a certain infrastructure dependency. The most important Aware interfaces
are summarized below - as a general rule, the name is a good indication of the
dependency type:

Note again that usage of these interfaces ties your code to the Spring API and does not
follow the Inversion of Control style. As such, they are recommended for infrastructure
beans that require programmatic access to the container.

7.7 Bean definition inheritance

A bean definition can contain a lot of configuration information, including constructor
arguments, property values, and container-specific information such as initialization
method, static factory method name, and so on. A child bean definition inherits
configuration data from a parent definition. The child definition can override some
values, or add others, as needed. Using parent and child bean definitions can save a lot
of typing. Effectively, this is a form of templating.

If you work with an ApplicationContext interface programmatically, child bean
definitions are represented by the ChildBeanDefinition class. Most users do not work
with them on this level, instead configuring bean definitions declaratively in something
like the ClassPathXmlApplicationContext. When you use XML-based configuration
metadata, you indicate a child bean definition by using the parent attribute,
specifying the parent bean as the value of this attribute.

<beanid="inheritedTestBean"abstract="true"class="org.springframework.beans.TestBean"><propertyname="name"value="parent"/><propertyname="age"value="1"/></bean><beanid="inheritsWithDifferentClass"class="org.springframework.beans.DerivedTestBean"parent="inheritedTestBean" init-method="initialize">
<propertyname="name"value="override"/><!-- the age property value of 1 will be inherited from parent --></bean>

A child bean definition uses the bean class from the parent definition if none is
specified, but can also override it. In the latter case, the child bean class must be
compatible with the parent, that is, it must accept the parent’s property values.

The preceding example explicitly marks the parent bean definition as abstract by using
the abstract attribute. If the parent definition does not specify a class, explicitly
marking the parent bean definition as abstract is required, as follows:

<beanid="inheritedTestBeanWithoutClass"abstract="true"><propertyname="name"value="parent"/><propertyname="age"value="1"/></bean><beanid="inheritsWithClass"class="org.springframework.beans.DerivedTestBean"parent="inheritedTestBeanWithoutClass"init-method="initialize"><propertyname="name"value="override"/><!-- age will inherit the value of 1 from the parent bean definition--></bean>

The parent bean cannot be instantiated on its own because it is incomplete, and it is
also explicitly marked as abstract. When a definition is abstract like this, it is
usable only as a pure template bean definition that serves as a parent definition for
child definitions. Trying to use such an abstract parent bean on its own, by referring
to it as a ref property of another bean or doing an explicit getBean() call with the
parent bean id, returns an error. Similarly, the container’s internal
preInstantiateSingletons() method ignores bean definitions that are defined as
abstract.

Note

ApplicationContext pre-instantiates all singletons by default. Therefore, it is
important (at least for singleton beans) that if you have a (parent) bean definition
which you intend to use only as a template, and this definition specifies a class, you
must make sure to set the abstract attribute to true, otherwise the application
context will actually (attempt to) pre-instantiate the abstract bean.

7.8 Container Extension Points

Typically, an application developer does not need to subclass ApplicationContext
implementation classes. Instead, the Spring IoC container can be extended by plugging in
implementations of special integration interfaces. The next few sections describe these
integration interfaces.

7.8.1 Customizing beans using a BeanPostProcessor

The BeanPostProcessor interface defines callback methods that you can implement to
provide your own (or override the container’s default) instantiation logic,
dependency-resolution logic, and so forth. If you want to implement some custom logic
after the Spring container finishes instantiating, configuring, and initializing a bean,
you can plug in one or more BeanPostProcessor implementations.

You can configure multiple BeanPostProcessor instances, and you can control the order
in which these BeanPostProcessors execute by setting the order property. You can
set this property only if the BeanPostProcessor implements the Ordered interface; if
you write your own BeanPostProcessor you should consider implementing the Ordered
interface too. For further details, consult the javadocs of the BeanPostProcessor and
Ordered interfaces. See also the note below on
programmatic
registration of BeanPostProcessors.

Note

BeanPostProcessors operate on bean (or object) instances; that is to say, the
Spring IoC container instantiates a bean instance and thenBeanPostProcessors do
their work.

BeanPostProcessors are scoped per-container. This is only relevant if you are
using container hierarchies. If you define a BeanPostProcessor in one container, it
will only post-process the beans in that container. In other words, beans that are
defined in one container are not post-processed by a BeanPostProcessor defined in
another container, even if both containers are part of the same hierarchy.

The org.springframework.beans.factory.config.BeanPostProcessor interface consists of
exactly two callback methods. When such a class is registered as a post-processor with
the container, for each bean instance that is created by the container, the
post-processor gets a callback from the container both before container
initialization methods (such as InitializingBean’s afterPropertiesSet() and any
declared init method) are called as well as after any bean initialization callbacks.
The post-processor can take any action with the bean instance, including ignoring the
callback completely. A bean post-processor typically checks for callback interfaces or
may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as
bean post-processors in order to provide proxy-wrapping logic.

An ApplicationContextautomatically detects any beans that are defined in the
configuration metadata which implement the BeanPostProcessor interface. The
ApplicationContext registers these beans as post-processors so that they can be called
later upon bean creation. Bean post-processors can be deployed in the container just
like any other beans.

Note that when declaring a BeanPostProcessor using an @Bean factory method on a
configuration class, the return type of the factory method should be the implementation
class itself or at least the org.springframework.beans.factory.config.BeanPostProcessor
interface, clearly indicating the post-processor nature of that bean. Otherwise, the
ApplicationContext won’t be able to autodetect it by type before fully creating it.
Since a BeanPostProcessor needs to be instantiated early in order to apply to the
initialization of other beans in the context, this early type detection is critical.

Programmatically registering BeanPostProcessors

While the recommended approach for BeanPostProcessor registration is through
ApplicationContext auto-detection (as described above), it is also possible to
register them programmatically against a ConfigurableBeanFactory using the
addBeanPostProcessor method. This can be useful when needing to evaluate conditional
logic before registration, or even for copying bean post processors across contexts in a
hierarchy. Note however that BeanPostProcessors added programmatically do not
respect the Ordered interface. Here it is the order of registration that
dictates the order of execution. Note also that BeanPostProcessors registered
programmatically are always processed before those registered through auto-detection,
regardless of any explicit ordering.

BeanPostProcessors and AOP auto-proxying

Classes that implement the BeanPostProcessor interface are special and are treated
differently by the container. All BeanPostProcessors and beans that they reference
directly are instantiated on startup, as part of the special startup phase of the
ApplicationContext. Next, all BeanPostProcessors are registered in a sorted fashion
and applied to all further beans in the container. Because AOP auto-proxying is
implemented as a BeanPostProcessor itself, neither BeanPostProcessors nor the beans
they reference directly are eligible for auto-proxying, and thus do not have aspects
woven into them.

For any such bean, you should see an informational log message: "Bean foo is not
eligible for getting processed by all BeanPostProcessor interfaces (for example: not
eligible for auto-proxying)".

Note that if you have beans wired into your BeanPostProcessor using autowiring or
@Resource (which may fall back to autowiring), Spring might access unexpected beans
when searching for type-matching dependency candidates, and therefore make them
ineligible for auto-proxying or other kinds of bean post-processing. For example, if you
have a dependency annotated with @Resource where the field/setter name does not
directly correspond to the declared name of a bean and no name attribute is used, then
Spring will access other beans for matching them by type.

The following examples show how to write, register, and use BeanPostProcessors in an
ApplicationContext.

Example: Hello World, BeanPostProcessor-style

This first example illustrates basic usage. The example shows a custom
BeanPostProcessor implementation that invokes the toString() method of each bean as
it is created by the container and prints the resulting string to the system console.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:lang="http://www.springframework.org/schema/lang"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang
http://www.springframework.org/schema/lang/spring-lang.xsd"><lang:groovyid="messenger"script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy"><lang:propertyname="message"value="Fiona Apple Is Just So Dreamy."/></lang:groovy><!--
when the above bean (messenger) is instantiated, this custom
BeanPostProcessor implementation will output the fact to the system console
--><beanclass="scripting.InstantiationTracingBeanPostProcessor"/></beans>

Notice how the InstantiationTracingBeanPostProcessor is simply defined. It does not
even have a name, and because it is a bean it can be dependency-injected just like any
other bean. (The preceding configuration also defines a bean that is backed by a Groovy
script. The Spring dynamic language support is detailed in the chapter entitled
Chapter 35, Dynamic language support.)

The following simple Java application executes the preceding code and configuration:

Example: The RequiredAnnotationBeanPostProcessor

Using callback interfaces or annotations in conjunction with a custom
BeanPostProcessor implementation is a common means of extending the Spring IoC
container. An example is Spring’s RequiredAnnotationBeanPostProcessor - a
BeanPostProcessor implementation that ships with the Spring distribution which ensures
that JavaBean properties on beans that are marked with an (arbitrary) annotation are
actually (configured to be) dependency-injected with a value.

The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor. The semantics of
this interface are similar to those of the BeanPostProcessor, with one major
difference: BeanFactoryPostProcessor operates on the bean configuration metadata;
that is, the Spring IoC container allows a BeanFactoryPostProcessor to read the
configuration metadata and potentially change it before the container instantiates
any beans other than BeanFactoryPostProcessors.

You can configure multiple BeanFactoryPostProcessors, and you can control the order in
which these BeanFactoryPostProcessors execute by setting the order property.
However, you can only set this property if the BeanFactoryPostProcessor implements the
Ordered interface. If you write your own BeanFactoryPostProcessor, you should
consider implementing the Ordered interface too. Consult the javadocs of the
BeanFactoryPostProcessor and Ordered interfaces for more details.

Note

If you want to change the actual bean instances (i.e., the objects that are created
from the configuration metadata), then you instead need to use a BeanPostProcessor
(described above in Section 7.8.1, “Customizing beans using a BeanPostProcessor”). While it is technically possible
to work with bean instances within a BeanFactoryPostProcessor (e.g., using
BeanFactory.getBean()), doing so causes premature bean instantiation, violating the
standard container lifecycle. This may cause negative side effects such as bypassing
bean post processing.

Also, BeanFactoryPostProcessors are scoped per-container. This is only relevant if
you are using container hierarchies. If you define a BeanFactoryPostProcessor in one
container, it will only be applied to the bean definitions in that container. Bean
definitions in one container will not be post-processed by BeanFactoryPostProcessors
in another container, even if both containers are part of the same hierarchy.

A bean factory post-processor is executed automatically when it is declared inside an
ApplicationContext, in order to apply changes to the configuration metadata that
define the container. Spring includes a number of predefined bean factory
post-processors, such as PropertyOverrideConfigurer and
PropertyPlaceholderConfigurer. A custom BeanFactoryPostProcessor can also be used,
for example, to register custom property editors.

An ApplicationContext automatically detects any beans that are deployed into it that
implement the BeanFactoryPostProcessor interface. It uses these beans as bean factory
post-processors, at the appropriate time. You can deploy these post-processor beans as
you would any other bean.

Note

As with BeanPostProcessors , you typically do not want to configure
BeanFactoryPostProcessors for lazy initialization. If no other bean references a
Bean(Factory)PostProcessor, that post-processor will not get instantiated at all.
Thus, marking it for lazy initialization will be ignored, and the
Bean(Factory)PostProcessor will be instantiated eagerly even if you set the
default-lazy-init attribute to true on the declaration of your <beans /> element.

Example: the Class name substitution PropertyPlaceholderConfigurer

You use the PropertyPlaceholderConfigurer to externalize property values from a bean
definition in a separate file using the standard Java Properties format. Doing so
enables the person deploying an application to customize environment-specific properties
such as database URLs and passwords, without the complexity or risk of modifying the
main XML definition file or files for the container.

Consider the following XML-based configuration metadata fragment, where a DataSource
with placeholder values is defined. The example shows properties configured from an
external Properties file. At runtime, a PropertyPlaceholderConfigurer is applied to
the metadata that will replace some properties of the DataSource. The values to replace
are specified as placeholders of the form ${property-name} which follows the Ant /
log4j / JSP EL style.

Therefore, the string ${jdbc.username} is replaced at runtime with the value 'sa', and
the same applies for other placeholder values that match keys in the properties file.
The PropertyPlaceholderConfigurer checks for placeholders in most properties and
attributes of a bean definition. Furthermore, the placeholder prefix and suffix can be
customized.

With the context namespace introduced in Spring 2.5, it is possible to configure
property placeholders with a dedicated configuration element. One or more locations can
be provided as a comma-separated list in the location attribute.

The PropertyPlaceholderConfigurer not only looks for properties in the Properties
file you specify. By default it also checks against the Java System properties if it
cannot find a property in the specified properties files. You can customize this
behavior by setting the systemPropertiesMode property of the configurer with one of
the following three supported integer values:

never (0): Never check system properties

fallback (1): Check system properties if not resolvable in the specified
properties files. This is the default.

override (2): Check system properties first, before trying the specified
properties files. This allows system properties to override any other property source.

Consult the PropertyPlaceholderConfigurer javadocs for more information.

Tip

You can use the PropertyPlaceholderConfigurer to substitute class names, which is
sometimes useful when you have to pick a particular implementation class at runtime. For
example:

If the class cannot be resolved at runtime to a valid class, resolution of the bean
fails when it is about to be created, which is during the preInstantiateSingletons()
phase of an ApplicationContext for a non-lazy-init bean.

Example: the PropertyOverrideConfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, resembles the
PropertyPlaceholderConfigurer, but unlike the latter, the original definitions can
have default values or no values at all for bean properties. If an overriding
Properties file does not have an entry for a certain bean property, the default
context definition is used.

Note that the bean definition is not aware of being overridden, so it is not
immediately obvious from the XML definition file that the override configurer is being
used. In case of multiple PropertyOverrideConfigurer instances that define different
values for the same bean property, the last one wins, due to the overriding mechanism.

This example file can be used with a container definition that contains a bean called
dataSource, which has driver and url properties.

Compound property names are also supported, as long as every component of the path
except the final property being overridden is already non-null (presumably initialized
by the constructors). In this example…​

foo.fred.bob.sammy=123

the sammy property of the bob property of the fred property of the foo bean
is set to the scalar value 123.

Note

Specified override values are always literal values; they are not translated into
bean references. This convention also applies when the original value in the XML bean
definition specifies a bean reference.

With the context namespace introduced in Spring 2.5, it is possible to configure
property overriding with a dedicated configuration element:

<context:property-overridelocation="classpath:override.properties"/>

7.8.3 Customizing instantiation logic with a FactoryBean

Implement the org.springframework.beans.factory.FactoryBean interface for objects that
are themselves factories.

The FactoryBean interface is a point of pluggability into the Spring IoC container’s
instantiation logic. If you have complex initialization code that is better expressed in
Java as opposed to a (potentially) verbose amount of XML, you can create your own
FactoryBean, write the complex initialization inside that class, and then plug your
custom FactoryBean into the container.

The FactoryBean interface provides three methods:

Object getObject(): returns an instance of the object this factory creates. The
instance can possibly be shared, depending on whether this factory returns singletons
or prototypes.

Class getObjectType(): returns the object type returned by the getObject() method
or null if the type is not known in advance.

The FactoryBean concept and interface is used in a number of places within the Spring
Framework; more than 50 implementations of the FactoryBean interface ship with Spring
itself.

When you need to ask a container for an actual FactoryBean instance itself instead of
the bean it produces, preface the bean’s id with the ampersand symbol ( &) when
calling the getBean() method of the ApplicationContext. So for a given FactoryBean
with an id of myBean, invoking getBean("myBean") on the container returns the
product of the FactoryBean; whereas, invoking getBean("&myBean") returns the
FactoryBean instance itself.

7.9 Annotation-based container configuration

Are annotations better than XML for configuring Spring?

The introduction of annotation-based configurations raised the question of whether this
approach is 'better' than XML. The short answer is it depends. The long answer is
that each approach has its pros and cons, and usually it is up to the developer to
decide which strategy suits them better. Due to the way they are defined, annotations
provide a lot of context in their declaration, leading to shorter and more concise
configuration. However, XML excels at wiring up components without touching their source
code or recompiling them. Some developers prefer having the wiring close to the source
while others argue that annotated classes are no longer POJOs and, furthermore, that the
configuration becomes decentralized and harder to control.

No matter the choice, Spring can accommodate both styles and even mix them together.
It’s worth pointing out that through its JavaConfig option, Spring allows
annotations to be used in a non-invasive way, without touching the target components
source code and that in terms of tooling, all configuration styles are supported by the
Spring Tool Suite.

An alternative to XML setups is provided by annotation-based configuration which rely on
the bytecode metadata for wiring up components instead of angle-bracket declarations.
Instead of using XML to describe a bean wiring, the developer moves the configuration
into the component class itself by using annotations on the relevant class, method, or
field declaration. As mentioned in the section called “Example: The RequiredAnnotationBeanPostProcessor”, using
a BeanPostProcessor in conjunction with annotations is a common means of extending the
Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing
required properties with the @Required annotation. Spring
2.5 made it possible to follow that same general approach to drive Spring’s dependency
injection. Essentially, the @Autowired annotation provides the same capabilities as
described in Section 7.4.5, “Autowiring collaborators” but with more fine-grained control and wider
applicability. Spring 2.5 also added support for JSR-250 annotations such as
@PostConstruct, and @PreDestroy. Spring 3.0 added support for JSR-330 (Dependency
Injection for Java) annotations contained in the javax.inject package such as @Inject
and @Named. Details about those annotations can be found in the
relevant section.

Note

Annotation injection is performed before XML injection, thus the latter
configuration will override the former for properties wired through both approaches.

As always, you can register them as individual bean definitions, but they can also be
implicitly registered by including the following tag in an XML-based Spring
configuration (notice the inclusion of the context namespace):

<context:annotation-config/> only looks for annotations on beans in the same
application context in which it is defined. This means that, if you put
<context:annotation-config/> in a WebApplicationContext for a DispatcherServlet,
it only checks for @Autowired beans in your controllers, and not your services. See
Section 22.2, “The DispatcherServlet” for more information.

7.9.1 @Required

The @Required annotation applies to bean property setter methods, as in the following
example:

This annotation simply indicates that the affected bean property must be populated at
configuration time, through an explicit property value in a bean definition or through
autowiring. The container throws an exception if the affected bean property has not been
populated; this allows for eager and explicit failure, avoiding NullPointerExceptions
or the like later on. It is still recommended that you put assertions into the bean
class itself, for example, into an init method. Doing so enforces those required
references and values even when you use the class outside of a container.

7.9.2 @Autowired

Note

JSR 330’s @Inject annotation can be used in place of Spring’s @Autowired annotation
in the examples below. See here for more details.

As of Spring Framework 4.3, the @Autowired constructor is no longer necessary if the
target bean only defines one constructor. If several constructors are available, at
least one must be annotated to teach the container which one it has to use.

As expected, you can also apply the @Autowired annotation to "traditional" setter
methods:

Your beans can implement the org.springframework.core.Ordered interface or either use
the @Order or standard @Priority annotation if you want items in the array or list
to be sorted into a specific order.

Even typed Maps can be autowired as long as the expected key type is String. The Map
values will contain all beans of the expected type, and the keys will contain the
corresponding bean names:

By default, the autowiring fails whenever zero candidate beans are available; the
default behavior is to treat annotated methods, constructors, and fields as
indicating required dependencies. This behavior can be changed as demonstrated below.

Only one annotated constructor per-class can be marked as required, but multiple
non-required constructors can be annotated. In that case, each is considered among the
candidates and Spring uses the greediest constructor whose dependencies can be
satisfied, that is the constructor that has the largest number of arguments.

@Autowired’s required attribute is recommended over the [email protected] annotation.
The required attribute indicates that the property is not required for autowiring
purposes, the property is ignored if it cannot be autowired. @Required, on the other
hand, is stronger in that it enforces the property that was set by any means supported
by the container. If no value is injected, a corresponding exception is raised.

You can also use @Autowired for interfaces that are well-known resolvable
dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader,
ApplicationEventPublisher, and MessageSource. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are
automatically resolved, with no special setup necessary.

@Autowired, @Inject, @Resource, and @Value annotations are handled by Spring
BeanPostProcessor implementations which in turn means that you cannot apply these
annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if
any). These types must be 'wired up' explicitly via XML or using a Spring @Bean method.

7.9.3 Fine-tuning annotation-based autowiring with @Primary

Because autowiring by type may lead to multiple candidates, it is often necessary to have
more control over the selection process. One way to accomplish this is with Spring’s
@Primary annotation. @Primary indicates that a particular bean should be given
preference when multiple beans are candidates to be autowired to a single-valued
dependency. If exactly one 'primary' bean exists among the candidates, it will be the
autowired value.

Let’s assume we have the following configuration that defines firstMovieCatalog as the
primaryMovieCatalog.

7.9.4 Fine-tuning annotation-based autowiring with qualifiers

@Primary is an effective way to use autowiring by type with several instances when one
primary candidate can be determined. When more control over the selection process is
required, Spring’s @Qualifier annotation can be used. You can associate qualifier values
with specific arguments, narrowing the set of type matches so that a specific bean is
chosen for each argument. In the simplest case, this can be a plain descriptive value:

For a fallback match, the bean name is considered a default qualifier value. Thus you
can define the bean with an id "main" instead of the nested qualifier element, leading
to the same matching result. However, although you can use this convention to refer to
specific beans by name, @Autowired is fundamentally about type-driven injection with
optional semantic qualifiers. This means that qualifier values, even with the bean name
fallback, always have narrowing semantics within the set of type matches; they do not
semantically express a reference to a unique bean id. Good qualifier values are "main"
or "EMEA" or "persistent", expressing characteristics of a specific component that are
independent from the bean id, which may be auto-generated in case of an anonymous bean
definition like the one in the preceding example.

Qualifiers also apply to typed collections, as discussed above, for example, to
Set<MovieCatalog>. In this case, all matching beans according to the declared
qualifiers are injected as a collection. This implies that qualifiers do not have to be
unique; they rather simply constitute filtering criteria. For example, you can define
multiple MovieCatalog beans with the same qualifier value "action", all of which would
be injected into a Set<MovieCatalog> annotated with @Qualifier("action").

Tip

If you intend to express annotation-driven injection by name, do not primarily use
@Autowired, even if is technically capable of referring to a bean name through
@Qualifier values. Instead, use the JSR-250 @Resource annotation, which is
semantically defined to identify a specific target component by its unique name, with
the declared type being irrelevant for the matching process. @Autowired has rather
different semantics: After selecting candidate beans by type, the specified String
qualifier value will be considered within those type-selected candidates only, e.g.
matching an "account" qualifier against beans marked with the same qualifier label.

For beans that are themselves defined as a collection/map or array type, @Resource
is a fine solution, referring to the specific collection or array bean by unique name.
That said, as of 4.3, collection/map and array types can be matched through Spring’s
@Autowired type matching algorithm as well, as long as the element type information
is preserved in @Bean return type signatures or collection inheritance hierarchies.
In this case, qualifier values can be used to select among same-typed collections,
as outlined in the previous paragraph.

As of 4.3, @Autowired also considers self references for injection, i.e. references
back to the bean that is currently injected. Note that self injection is a fallback;
regular dependencies on other components always have precedence. In that sense, self
references do not participate in regular candidate selection and are therefore in
particular never primary; on the contrary, they always end up as lowest precedence.
In practice, use self references as a last resort only, e.g. for calling other methods
on the same instance through the bean’s transactional proxy: Consider factoring out
the affected methods to a separate delegate bean in such a scenario. Alternatively,
use @Resource which may obtain a proxy back to the current bean by its unique name.

@Autowired applies to fields, constructors, and multi-argument methods, allowing for
narrowing through qualifier annotations at the parameter level. By contrast, @Resource
is supported only for fields and bean property setter methods with a single argument.
As a consequence, stick with qualifiers if your injection target is a constructor or a
multi-argument method.

You can create your own custom qualifier annotations. Simply define an annotation and
provide the @Qualifier annotation within your definition:

Next, provide the information for the candidate bean definitions. You can add
<qualifier/> tags as sub-elements of the <bean/> tag and then specify the type and
value to match your custom qualifier annotations. The type is matched against the
fully-qualified class name of the annotation. Or, as a convenience if no risk of
conflicting names exists, you can use the short class name. Both approaches are
demonstrated in the following example.

In some cases, it may be sufficient to use an annotation without a value. This may be
useful when the annotation serves a more generic purpose and can be applied across
several different types of dependencies. For example, you may provide an offline
catalog that would be searched when no Internet connection is available. First define
the simple annotation:

<beanclass="example.SimpleMovieCatalog"><qualifier type="Offline"/><!-- inject any dependencies required by this bean --></bean>

You can also define custom qualifier annotations that accept named attributes in
addition to or instead of the simple value attribute. If multiple attribute values are
then specified on a field or parameter to be autowired, a bean definition must match
all such attribute values to be considered an autowire candidate. As an example,
consider the following annotation definition:

Finally, the bean definitions should contain matching qualifier values. This example
also demonstrates that bean meta attributes may be used instead of the
<qualifier/> sub-elements. If available, the <qualifier/> and its attributes take
precedence, but the autowiring mechanism falls back on the values provided within the
<meta/> tags if no such qualifier is present, as in the last two bean definitions in
the following example.

<?xml version="1.0" encoding="UTF-8"?><beansxmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:context="http://www.springframework.org/schema/context"xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd"><context:annotation-config/><beanclass="example.SimpleMovieCatalog"><qualifiertype="MovieQualifier"><attributekey="format"value="VHS"/><attributekey="genre"value="Action"/></qualifier><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><qualifiertype="MovieQualifier"><attributekey="format"value="VHS"/><attributekey="genre"value="Comedy"/></qualifier><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><metakey="format"value="DVD"/><metakey="genre"value="Action"/><!-- inject any dependencies required by this bean --></bean><beanclass="example.SimpleMovieCatalog"><metakey="format"value="BLURAY"/><metakey="genre"value="Comedy"/><!-- inject any dependencies required by this bean --></bean></beans>

7.9.5 Using generics as autowiring qualifiers

In addition to the @Qualifier annotation, it is also possible to use Java generic types
as an implicit form of qualification. For example, suppose you have the following
configuration:

any default-autowire-candidates pattern(s) available on the <beans/> element

the presence of @Qualifier annotations and any custom annotations registered
with the CustomAutowireConfigurer

When multiple beans qualify as autowire candidates, the determination of a "primary" is
the following: if exactly one bean definition among the candidates has a primary
attribute set to true, it will be selected.

7.9.7 @Resource

Spring also supports injection using the JSR-250 @Resource annotation on fields or
bean property setter methods. This is a common pattern in Java EE 5 and 6, for example
in JSF 1.2 managed beans or JAX-WS 2.0 endpoints. Spring supports this pattern for
Spring-managed objects as well.

@Resource takes a name attribute, and by default Spring interprets that value as the
bean name to be injected. In other words, it follows by-name semantics, as
demonstrated in this example:

If no name is specified explicitly, the default name is derived from the field name or
setter method. In case of a field, it takes the field name; in case of a setter method,
it takes the bean property name. So the following example is going to have the bean with
name "movieFinder" injected into its setter method:

The name provided with the annotation is resolved as a bean name by the
ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The
names can be resolved through JNDI if you configure Spring’s
SimpleJndiBeanFactory
explicitly. However, it is recommended that you rely on the default behavior and simply
use Spring’s JNDI lookup capabilities to preserve the level of indirection.

In the exclusive case of @Resource usage with no explicit name specified, and similar
to @Autowired, @Resource finds a primary type match instead of a specific named bean
and resolves well-known resolvable dependencies: the BeanFactory,
ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource
interfaces.

Thus in the following example, the customerPreferenceDao field first looks for a bean
named customerPreferenceDao, then falls back to a primary type match for the type
CustomerPreferenceDao. The "context" field is injected based on the known resolvable
dependency type ApplicationContext.

7.9.8 @PostConstruct and @PreDestroy

The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation
but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support
for these annotations offers yet another alternative to those described in
initialization callbacks and
destruction callbacks. Provided that the
CommonAnnotationBeanPostProcessor is registered within the Spring
ApplicationContext, a method carrying one of these annotations is invoked at the same
point in the lifecycle as the corresponding Spring lifecycle interface method or
explicitly declared callback method. In the example below, the cache will be
pre-populated upon initialization and cleared upon destruction.

7.10 Classpath scanning and managed components

Most examples in this chapter use XML to specify the configuration metadata that produces
each BeanDefinition within the Spring container. The previous section
(Section 7.9, “Annotation-based container configuration”) demonstrates how to provide a lot of the configuration
metadata through source-level annotations. Even in those examples, however, the "base"
bean definitions are explicitly defined in the XML file, while the annotations only drive
the dependency injection. This section describes an option for implicitly detecting the
candidate components by scanning the classpath. Candidate components are classes that
match against a filter criteria and have a corresponding bean definition registered with
the container. This removes the need to use XML to perform bean registration; instead you
can use annotations (for example @Component), AspectJ type expressions, or your own
custom filter criteria to select which classes will have bean definitions registered with
the container.

Note

Starting with Spring 3.0, many features provided by the Spring JavaConfig project are
part of the core Spring Framework. This allows you to define beans using Java rather
than using the traditional XML files. Take a look at the @Configuration, @Bean,
@Import, and @DependsOn annotations for examples of how to use these new features.

7.10.1 @Component and further stereotype annotations

The @Repository annotation is a marker for any class that fulfills the role or
stereotype of a repository (also known as Data Access Object or DAO). Among the uses
of this marker is the automatic translation of exceptions as described in
Section 20.2.2, “Exception translation”.

Spring provides further stereotype annotations: @Component, @Service, and
@Controller. @Component is a generic stereotype for any Spring-managed component.
@Repository, @Service, and @Controller are specializations of @Component for
more specific use cases, for example, in the persistence, service, and presentation
layers, respectively. Therefore, you can annotate your component classes with
@Component, but by annotating them with @Repository, @Service, or @Controller
instead, your classes are more properly suited for processing by tools or associating
with aspects. For example, these stereotype annotations make ideal targets for
pointcuts. It is also possible that @Repository, @Service, and @Controller may
carry additional semantics in future releases of the Spring Framework. Thus, if you are
choosing between using @Component or @Service for your service layer, @Service is
clearly the better choice. Similarly, as stated above, @Repository is already
supported as a marker for automatic exception translation in your persistence layer.

7.10.2 Meta-annotations

Many of the annotations provided by Spring can be used as meta-annotations in your
own code. A meta-annotation is simply an annotation that can be applied to another
annotation. For example, the @Service annotation mentioned above is meta-annotated with
@Component:

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Component// Spring will see this and treat @Service in the same way as @Componentpublic@interface Service {
// ....
}

Meta-annotations can also be combined to create composed annotations. For example,
the @RestController annotation from Spring MVC is composed of @Controller and
@ResponseBody.

In addition, composed annotations may optionally redeclare attributes from
meta-annotations to allow user customization. This can be particularly useful when you
want to only expose a subset of the meta-annotation’s attributes. For example, Spring’s
@SessionScope annotation hardcodes the scope name to session but still allows
customization of the proxyMode.

To autodetect these classes and register the corresponding beans, you need to add
@ComponentScan to your @Configuration class, where the basePackages attribute
is a common parent package for the two classes. (Alternatively, you can specify a
comma/semicolon/space-separated list that includes the parent package of each class.)

The use of <context:component-scan> implicitly enables the functionality of
<context:annotation-config>. There is usually no need to include the
<context:annotation-config> element when using <context:component-scan>.

Note

The scanning of classpath packages requires the presence of corresponding directory
entries in the classpath. When you build JARs with Ant, make sure that you do not
activate the files-only switch of the JAR task. Also, classpath directories may not
get exposed based on security policies in some environments, e.g. standalone apps on
JDK 1.7.0_45 and higher (which requires 'Trusted-Library' setup in your manifests; see
http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources).

Furthermore, the AutowiredAnnotationBeanPostProcessor and
CommonAnnotationBeanPostProcessor are both included implicitly when you use the
component-scan element. That means that the two components are autodetected and
wired together - all without any bean configuration metadata provided in XML.

Note

You can disable the registration of AutowiredAnnotationBeanPostProcessor and
CommonAnnotationBeanPostProcessor by including the annotation-config attribute
with a value of false.

7.10.4 Using filters to customize scanning

By default, classes annotated with @Component, @Repository, @Service,
@Controller, or a custom annotation that itself is annotated with @Component are the
only detected candidate components. However, you can modify and extend this behavior
simply by applying custom filters. Add them as includeFilters or excludeFilters
parameters of the @ComponentScan annotation (or as include-filter or exclude-filter
sub-elements of the component-scan element). Each filter element requires the type
and expression attributes. The following table describes the filtering options.

Table 7.5. Filter Types

Filter Type

Example Expression

Description

annotation (default)

org.example.SomeAnnotation

An annotation to be present at the type level in target components.

assignable

org.example.SomeClass

A class (or interface) that the target components are assignable to (extend/implement).

aspectj

org.example..*Service+

An AspectJ type expression to be matched by the target components.

regex

org\.example\.Default.*

A regex expression to be matched by the target components class names.

custom

org.example.MyTypeFilter

A custom implementation of the org.springframework.core.type .TypeFilter interface.

The following example shows the configuration ignoring all @Repository annotations
and using "stub" repositories instead.

You can also disable the default filters by setting useDefaultFilters=false on the annotation or
providing use-default-filters="false" as an attribute of the <component-scan/> element. This
will in effect disable automatic detection of classes annotated with @Component, @Repository,
@Service, @Controller, or @Configuration.

7.10.5 Defining bean metadata within components

Spring components can also contribute bean definition metadata to the container. You do
this with the same @Bean annotation used to define bean metadata within @Configuration
annotated classes. Here is a simple example:

This class is a Spring component that has application-specific code contained in its
doWork() method. However, it also contributes a bean definition that has a factory
method referring to the method publicInstance(). The @Bean annotation identifies the
factory method and other bean definition properties, such as a qualifier value through
the @Qualifier annotation. Other method level annotations that can be specified are
@Scope, @Lazy, and custom qualifier annotations.

Tip

In addition to its role for component initialization, the @Lazy annotation may also be
placed on injection points marked with @Autowired or @Inject. In this context, it
leads to the injection of a lazy-resolution proxy.

Autowired fields and methods are supported as previously discussed, with additional
support for autowiring of @Bean methods:

The example autowires the String method parameter country to the value of the Age
property on another bean named privateInstance. A Spring Expression Language element
defines the value of the property through the notation #{ <expression> }. For @Value
annotations, an expression resolver is preconfigured to look for bean names when
resolving expression text.

As of Spring Framework 4.3, you may also declare a factory method parameter of type
InjectionPoint (or its more specific subclass DependencyDescriptor) in order to
access the requesting injection point that triggers the creation of the current bean.
Note that this will only apply to the actual creation of bean instances, not to the
injection of existing instances. As a consequence, this feature makes most sense for
beans of prototype scope. For other scopes, the factory method will only ever see the
injection point which triggered the creation of a new bean instance in the given scope:
for example, the dependency that triggered the creation of a lazy singleton bean.
Use the provided injection point metadata with semantic care in such scenarios.

The @Bean methods in a regular Spring component are processed differently than their
counterparts inside a Spring @Configuration class. The difference is that @Component
classes are not enhanced with CGLIB to intercept the invocation of methods and fields.
CGLIB proxying is the means by which invoking methods or fields within @Bean methods
in @Configuration classes creates bean metadata references to collaborating objects;
such methods are not invoked with normal Java semantics but rather go through the
container in order to provide the usual lifecycle management and proxying of Spring
beans even when referring to other beans via programmatic calls to @Bean methods.
In contrast, invoking a method or field in an @Bean method within a plain @Component
class has standard Java semantics, with no special CGLIB processing or other
constraints applying.

Note

You may declare @Bean methods as static, allowing for them to be called without
creating their containing configuration class as an instance. This makes particular
sense when defining post-processor beans, e.g. of type BeanFactoryPostProcessor or
BeanPostProcessor, since such beans will get initialized early in the container
lifecycle and should avoid triggering other parts of the configuration at that point.

Note that calls to static @Bean methods will never get intercepted by the container,
not even within @Configuration classes (see above). This is due to technical
limitations: CGLIB subclassing can only override non-static methods. As a consequence,
a direct call to another @Bean method will have standard Java semantics, resulting
in an independent instance being returned straight from the factory method itself.

The Java language visibility of @Bean methods does not have an immediate impact on
the resulting bean definition in Spring’s container. You may freely declare your
factory methods as you see fit in non-@Configuration classes and also for static
methods anywhere. However, regular @Bean methods in @Configuration classes need
to be overridable, i.e. they must not be declared as private or final.

@Bean methods will also be discovered on base classes of a given component or
configuration class, as well as on Java 8 default methods declared in interfaces
implemented by the component or configuration class. This allows for a lot of
flexibility in composing complex configuration arrangements, with even multiple
inheritance being possible through Java 8 default methods as of Spring 4.2.

Finally, note that a single class may hold multiple @Bean methods for the same
bean, as an arrangement of multiple factory methods to use depending on available
dependencies at runtime. This is the same algorithm as for choosing the "greediest"
constructor or factory method in other configuration scenarios: The variant with
the largest number of satisfiable dependencies will be picked at construction time,
analogous to how the container selects between multiple @Autowired constructors.

7.10.6 Naming autodetected components

When a component is autodetected as part of the scanning process, its bean name is
generated by the BeanNameGenerator strategy known to that scanner. By default, any
Spring stereotype annotation (@Component, @Repository, @Service, and
@Controller) that contains a namevalue will thereby provide that name to the
corresponding bean definition.

If such an annotation contains no namevalue or for any other detected component (such
as those discovered by custom filters), the default bean name generator returns the
uncapitalized non-qualified class name. For example, if the following two components
were detected, the names would be myMovieLister and movieFinderImpl:

If you do not want to rely on the default bean-naming strategy, you can provide a custom
bean-naming strategy. First, implement the
BeanNameGenerator
interface, and be sure to include a default no-arg constructor. Then, provide the
fully-qualified class name when configuring the scanner:

As a general rule, consider specifying the name with the annotation whenever other
components may be making explicit references to it. On the other hand, the
auto-generated names are adequate whenever the container is responsible for wiring.

7.10.7 Providing a scope for autodetected components

As with Spring-managed components in general, the default and most common scope for
autodetected components is singleton. However, sometimes you need a different scope
which can be specified via the @Scope annotation. Simply provide the name of the scope
within the annotation:

To provide a custom strategy for scope resolution rather than relying on the
annotation-based approach, implement the
ScopeMetadataResolver
interface, and be sure to include a default no-arg constructor. Then, provide the
fully-qualified class name when configuring the scanner:

When using certain non-singleton scopes, it may be necessary to generate proxies for the
scoped objects. The reasoning is described in the section called “Scoped beans as dependencies”.
For this purpose, a scoped-proxy attribute is available on the component-scan
element. The three possible values are: no, interfaces, and targetClass. For example,
the following configuration will result in standard JDK dynamic proxies:

7.10.8 Providing qualifier metadata with annotations

The @Qualifier annotation is discussed in Section 7.9.4, “Fine-tuning annotation-based autowiring with qualifiers”.
The examples in that section demonstrate the use of the @Qualifier annotation and
custom qualifier annotations to provide fine-grained control when you resolve autowire
candidates. Because those examples were based on XML bean definitions, the qualifier
metadata was provided on the candidate bean definitions using the qualifier or meta
sub-elements of the bean element in the XML. When relying upon classpath scanning for
autodetection of components, you provide the qualifier metadata with type-level
annotations on the candidate class. The following three examples demonstrate this
technique:

As with most annotation-based alternatives, keep in mind that the annotation metadata is
bound to the class definition itself, while the use of XML allows for multiple beans
of the same type to provide variations in their qualifier metadata, because that
metadata is provided per-instance rather than per-class.

7.11 Using JSR 330 Standard Annotations

Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations
(Dependency Injection). Those annotations are scanned in the same way as the Spring
annotations. You just need to have the relevant jars in your classpath.

As with @Autowired, it is possible to use @Inject at the field level, method level
and constructor-argument level. Furthermore, you may declare your injection point as a
Provider, allowing for on-demand access to beans of shorter scopes or lazy access to
other beans through a Provider.get() call. As a variant of the example above:

In contrast to @Component, the JSR-330 @Named and the JSR-250 ManagedBean
annotations are not composable. Please use Spring’s stereotype model for building custom
component annotations.

7.11.3 Limitations of JSR-330 standard annotations

When working with standard annotations, it is important to know that some significant
features are not available as shown in the table below:

Table 7.6. Spring component model elements vs. JSR-330 variants

Spring

javax.inject.*

javax.inject restrictions / comments

@Autowired

@Inject

@Inject has no 'required' attribute; can be used with Java 8’s Optional instead.

@Component

@Named / @ManagedBean

JSR-330 does not provide a composable model, just a way to identify named components.

@Scope("singleton")

@Singleton

The JSR-330 default scope is like Spring’s prototype. However, in order to keep it
consistent with Spring’s general defaults, a JSR-330 bean declared in the Spring
container is a singleton by default. In order to use a scope other than singleton,
you should use Spring’s @Scope annotation. javax.inject also provides a
@Scope annotation.
Nevertheless, this one is only intended to be used for creating your own annotations.

@Qualifier

@Qualifier / @Named

javax.inject.Qualifier is just a meta-annotation for building custom qualifiers.
Concrete String qualifiers (like Spring’s @Qualifier with a value) can be associated
through javax.inject.Named.

@Value

-

no equivalent

@Required

-

no equivalent

@Lazy

-

no equivalent

ObjectFactory

Provider

javax.inject.Provider is a direct alternative to Spring’s ObjectFactory,
just with a shorter get() method name. It can also be used in combination with
Spring’s @Autowired or with non-annotated constructors and setter methods.

7.12 Java-based container configuration

7.12.1 Basic concepts: @Bean and @Configuration

The central artifacts in Spring’s new Java-configuration support are
@Configuration-annotated classes and @Bean-annotated methods.

The @Bean annotation is used to indicate that a method instantiates, configures and
initializes a new object to be managed by the Spring IoC container. For those familiar
with Spring’s <beans/> XML configuration the @Bean annotation plays the same role as
the <bean/> element. You can use @Bean annotated methods with any Spring
@Component, however, they are most often used with @Configuration beans.

Annotating a class with @Configuration indicates that its primary purpose is as a
source of bean definitions. Furthermore, @Configuration classes allow inter-bean
dependencies to be defined by simply calling other @Bean methods in the same class.
The simplest possible @Configuration class would read as follows:

When @Bean methods are declared within classes that are not annotated with
@Configuration they are referred to as being processed in a 'lite' mode. For example,
bean methods declared in a @Component or even in a plain old class will be
considered 'lite'.

Unlike full @Configuration, lite @Bean methods cannot easily declare inter-bean
dependencies. Usually one @Bean method should not invoke another @Bean method when
operating in 'lite' mode.

Only using @Bean methods within @Configuration classes is a recommended approach of
ensuring that 'full' mode is always used. This will prevent the same @Bean method from
accidentally being invoked multiple times and helps to reduce subtle bugs that can be
hard to track down when operating in 'lite' mode.

The @Bean and @Configuration annotations will be discussed in depth in the sections
below. First, however, we’ll cover the various ways of creating a spring container using
Java-based configuration.

The sections below document Spring’s AnnotationConfigApplicationContext, new in Spring
3.0. This versatile ApplicationContext implementation is capable of accepting not only
@Configuration classes as input, but also plain @Component classes and classes
annotated with JSR-330 metadata.

When @Configuration classes are provided as input, the @Configuration class itself
is registered as a bean definition, and all declared @Bean methods within the class
are also registered as bean definitions.

When @Component and JSR-330 classes are provided, they are registered as bean
definitions, and it is assumed that DI metadata such as @Autowired or @Inject are
used within those classes where necessary.

Simple construction

In much the same way that Spring XML files are used as input when instantiating a
ClassPathXmlApplicationContext, @Configuration classes may be used as input when
instantiating an AnnotationConfigApplicationContext. This allows for completely
XML-free usage of the Spring container:

As mentioned above, AnnotationConfigApplicationContext is not limited to working only
with @Configuration classes. Any @Component or JSR-330 annotated class may be supplied
as input to the constructor. For example:

The above assumes that MyServiceImpl, Dependency1 and Dependency2 use Spring
dependency injection annotations such as @Autowired.

Building the container programmatically using register(Class<?>…​)

An AnnotationConfigApplicationContext may be instantiated using a no-arg constructor
and then configured using the register() method. This approach is particularly useful
when programmatically building an AnnotationConfigApplicationContext.

Enabling component scanning with scan(String…​)

Experienced Spring users will be familiar with the XML declaration equivalent from
Spring’s context: namespace

<beans><context:component-scanbase-package="com.acme"/></beans>

In the example above, the com.acme package will be scanned, looking for any
@Component-annotated classes, and those classes will be registered as Spring bean
definitions within the container. AnnotationConfigApplicationContext exposes the
scan(String…​) method to allow for the same component-scanning functionality:

Remember that @Configuration classes are meta-annotated
with @Component, so they are candidates for component-scanning! In the example above,
assuming that AppConfig is declared within the com.acme package (or any package
underneath), it will be picked up during the call to scan(), and upon refresh() all
its @Bean methods will be processed and registered as bean definitions within the
container.

Support for web applications with AnnotationConfigWebApplicationContext

A WebApplicationContext variant of AnnotationConfigApplicationContext is available
with AnnotationConfigWebApplicationContext. This implementation may be used when
configuring the Spring ContextLoaderListener servlet listener, Spring MVC
DispatcherServlet, etc. What follows is a web.xml snippet that configures a typical
Spring MVC web application. Note the use of the contextClass context-param and
init-param:

<web-app><!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext --><context-param><param-name>contextClass</param-name><param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value></context-param><!-- Configuration locations must consist of one or more comma- or space-delimited
fully-qualified @Configuration classes. Fully-qualified packages may also be
specified for component-scanning --><context-param><param-name>contextConfigLocation</param-name><param-value>com.acme.AppConfig</param-value></context-param><!-- Bootstrap the root application context as usual using ContextLoaderListener --><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener><!-- Declare a Spring MVC DispatcherServlet as usual --><servlet><servlet-name>dispatcher</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext
instead of the default XmlWebApplicationContext --><init-param><param-name>contextClass</param-name><param-value>
org.springframework.web.context.support.AnnotationConfigWebApplicationContext
</param-value></init-param><!-- Again, config locations must consist of one or more comma- or space-delimited
and fully-qualified @Configuration classes --><init-param><param-name>contextConfigLocation</param-name><param-value>com.acme.web.MvcConfig</param-value></init-param></servlet><!-- map all requests for /app/* to the dispatcher servlet --><servlet-mapping><servlet-name>dispatcher</servlet-name><url-pattern>/app/*</url-pattern></servlet-mapping></web-app>

7.12.3 Using the @Bean annotation

@Bean is a method-level annotation and a direct analog of the XML <bean/> element.
The annotation supports some of the attributes offered by <bean/>, such as:
init-method,
destroy-method,
autowiring and name.

You can use the @Bean annotation in a @Configuration-annotated or in a
@Component-annotated class.

Declaring a bean

To declare a bean, simply annotate a method with the @Bean annotation. You use this
method to register a bean definition within an ApplicationContext of the type
specified as the method’s return value. By default, the bean name will be the same as
the method name. The following is a simple example of a @Bean method declaration:

Both declarations make a bean named transferService available in the
ApplicationContext, bound to an object instance of type TransferServiceImpl:

transferService -> com.acme.TransferServiceImpl

Bean dependencies

A @Bean annotated method can have an arbitrary number of parameters describing the
dependencies required to build that bean. For instance if our TransferService
requires an AccountRepository we can materialize that dependency via a method
parameter:

The resolution mechanism is pretty much identical to constructor-based dependency
injection, see the relevant section for more details.

Receiving lifecycle callbacks

Any classes defined with the @Bean annotation support the regular lifecycle callbacks
and can use the @PostConstruct and @PreDestroy annotations from JSR-250, see
JSR-250 annotations for further
details.

The regular Spring lifecycle callbacks are fully supported as
well. If a bean implements InitializingBean, DisposableBean, or Lifecycle, their
respective methods are called by the container.

By default, beans defined using Java config that have a public close or shutdown
method are automatically enlisted with a destruction callback. If you have a public
close or shutdown method and you do not wish for it to be called when the container
shuts down, simply add @Bean(destroyMethod="") to your bean definition to disable the
default (inferred) mode.

You may want to do that by default for a resource that you acquire via JNDI as its
lifecycle is managed outside the application. In particular, make sure to always do it
for a DataSource as it is known to be problematic on Java EE application servers.

Also, with @Bean methods, you will typically choose to use programmatic JNDI lookups:
either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or straight JNDI
InitialContext usage, but not the JndiObjectFactoryBean variant which would force
you to declare the return type as the FactoryBean type instead of the actual target
type, making it harder to use for cross-reference calls in other @Bean methods that
intend to refer to the provided resource here.

Of course, in the case of Foo above, it would be equally as valid to call the init()
method directly during construction:

@Scope and scoped-proxy

Spring offers a convenient way of working with scoped dependencies through
scoped proxies. The easiest way to create such
a proxy when using the XML configuration is the <aop:scoped-proxy/> element.
Configuring your beans in Java with a @Scope annotation offers equivalent support with
the proxyMode attribute. The default is no proxy ( ScopedProxyMode.NO), but you can
specify ScopedProxyMode.TARGET_CLASS or ScopedProxyMode.INTERFACES.

If you port the scoped proxy example from the XML reference documentation (see preceding
link) to our @Bean using Java, it would look like the following:

Bean aliasing

As discussed in Section 7.3.1, “Naming beans”, it is sometimes desirable to give a single bean
multiple names, otherwise known as bean aliasing. The name attribute of the @Bean
annotation accepts a String array for this purpose.

7.12.4 Using the @Configuration annotation

@Configuration is a class-level annotation indicating that an object is a source of
bean definitions. @Configuration classes declare beans via public @Bean annotated
methods. Calls to @Bean methods on @Configuration classes can also be used to define
inter-bean dependencies. See Section 7.12.1, “Basic concepts: @Bean and @Configuration” for a general introduction.

Injecting inter-bean dependencies

When @Beans have dependencies on one another, expressing that dependency is as simple
as having one bean method call another:

In the example above, the foo bean receives a reference to bar via constructor
injection.

Note

This method of declaring inter-bean dependencies only works when the @Bean method is
declared within a @Configuration class. You cannot declare inter-bean dependencies
using plain @Component classes.

Lookup method injection

As noted earlier, lookup method injection is an
advanced feature that you should use rarely. It is useful in cases where a
singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this
type of configuration provides a natural means for implementing this pattern.

publicabstractclass CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?protectedabstract Command createCommand();
}

Using Java-configuration support , you can create a subclass of CommandManager where
the abstract createCommand() method is overridden in such a way that it looks up a new
(prototype) command object:

clientDao() has been called once in clientService1() and once in clientService2().
Since this method creates a new instance of ClientDaoImpl and returns it, you would
normally expect having 2 instances (one for each service). That definitely would be
problematic: in Spring, instantiated beans have a singleton scope by default. This is
where the magic comes in: All @Configuration classes are subclassed at startup-time
with CGLIB. In the subclass, the child method checks the container first for any
cached (scoped) beans before it calls the parent method and creates a new instance. Note
that as of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because
CGLIB classes have been repackaged under org.springframework.cglib and included directly
within the spring-core JAR.

Note

The behavior could be different according to the scope of your bean. We are talking
about singletons here.

Tip

There are a few restrictions due to the fact that CGLIB dynamically adds features at
startup-time, in particular that configuration classes must not be final. However, as
of 4.3, any constructors are allowed on configuration classes, including the use of
@Autowired or a single non-default constructor declaration for default injection.

If you prefer to avoid any CGLIB-imposed limitations, consider declaring your @Bean
methods on non-@Configuration classes, e.g. on plain @Component classes instead.
Cross-method calls between @Bean methods won’t get intercepted then, so you’ll have
to exclusively rely on dependency injection at the constructor or method level there.

7.12.5 Composing Java-based configurations

Using the @Import annotation

Much as the <import/> element is used within Spring XML files to aid in modularizing
configurations, the @Import annotation allows for loading @Bean definitions from
another configuration class:

Now, rather than needing to specify both ConfigA.class and ConfigB.class when
instantiating the context, only ConfigB needs to be supplied explicitly:

publicstaticvoid main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);
// now both beans A and B will be available...
A a = ctx.getBean(A.class);
B b = ctx.getBean(B.class);
}

This approach simplifies container instantiation, as only one class needs to be dealt
with, rather than requiring the developer to remember a potentially large number of
@Configuration classes during construction.

Tip

As of Spring Framework 4.2, @Import also supports references to regular component
classes, analogous to the AnnotationConfigApplicationContext.register method.
This is particularly useful if you’d like to avoid component scanning, using a few
configuration classes as entry points for explicitly defining all your components.

Injecting dependencies on imported @Bean definitions

The example above works, but is simplistic. In most practical scenarios, beans will have
dependencies on one another across configuration classes. When using XML, this is not an
issue, per se, because there is no compiler involved, and one can simply declare
ref="someBean" and trust that Spring will work it out during container initialization.
Of course, when using @Configuration classes, the Java compiler places constraints on
the configuration model, in that references to other beans must be valid Java syntax.

Fortunately, solving this problem is simple. As we already discussed,
@Bean method can have an arbitrary number of parameters describing the bean
dependencies. Let’s consider a more real-world scenario with several @Configuration
classes, each depending on beans declared in the others:

There is another way to achieve the same result. Remember that @Configuration classes are
ultimately just another bean in the container: This means that they can take advantage of
@Autowired and @Value injection etc just like any other bean!

Warning

Make sure that the dependencies you inject that way are of the simplest kind only. @Configuration
classes are processed quite early during the initialization of the context and forcing a dependency
to be injected this way may lead to unexpected early initialization. Whenever possible, resort to
parameter-based injection as in the example above.

Also, be particularly careful with BeanPostProcessor and BeanFactoryPostProcessor definitions
via @Bean. Those should usually be declared as static @Bean methods, not triggering the
instantiation of their containing configuration class. Otherwise, @Autowired and @Value won’t
work on the configuration class itself since it is being created as a bean instance too early.

Constructor injection in @Configuration classes is only supported as of Spring
Framework 4.3. Note also that there is no need to specify @Autowired if the target
bean defines only one constructor; in the example above, @Autowired is not necessary
on the RepositoryConfig constructor.

In the scenario above, using @Autowired works well and provides the desired
modularity, but determining exactly where the autowired bean definitions are declared is
still somewhat ambiguous. For example, as a developer looking at ServiceConfig, how do
you know exactly where the @Autowired AccountRepository bean is declared? It’s not
explicit in the code, and this may be just fine. Remember that the
Spring Tool Suite provides tooling that
can render graphs showing how everything is wired up - that may be all you need. Also,
your Java IDE can easily find all declarations and uses of the AccountRepository type,
and will quickly show you the location of @Bean methods that return that type.

In cases where this ambiguity is not acceptable and you wish to have direct navigation
from within your IDE from one @Configuration class to another, consider autowiring the
configuration classes themselves:

In the situation above, it is completely explicit where AccountRepository is defined.
However, ServiceConfig is now tightly coupled to RepositoryConfig; that’s the
tradeoff. This tight coupling can be somewhat mitigated by using interface-based or
abstract class-based @Configuration classes. Consider the following:

Now ServiceConfig is loosely coupled with respect to the concrete
DefaultRepositoryConfig, and built-in IDE tooling is still useful: it will be easy for
the developer to get a type hierarchy of RepositoryConfig implementations. In this
way, navigating @Configuration classes and their dependencies becomes no different
than the usual process of navigating interface-based code.

Conditionally include @Configuration classes or @Bean methods

It is often useful to conditionally enable or disable a complete @Configuration class,
or even individual @Bean methods, based on some arbitrary system state. One common
example of this is to use the @Profile annotation to activate beans only when a specific
profile has been enabled in the Spring Environment (see Section 7.13.1, “Bean definition profiles”
for details).

The @Profile annotation is actually implemented using a much more flexible annotation
called @Conditional.
The @Conditional annotation indicates specific
org.springframework.context.annotation.Condition implementations that should be
consulted before a @Bean is registered.

Implementations of the Condition interface simply provide a matches(…​)
method that returns true or false. For example, here is the actual
Condition implementation used for @Profile:

Combining Java and XML configuration

Spring’s @Configuration class support does not aim to be a 100% complete replacement
for Spring XML. Some facilities such as Spring XML namespaces remain an ideal way to
configure the container. In cases where XML is convenient or necessary, you have a
choice: either instantiate the container in an "XML-centric" way using, for example,
ClassPathXmlApplicationContext, or in a "Java-centric" fashion using
AnnotationConfigApplicationContext and the @ImportResource annotation to import XML
as needed.

XML-centric use of @Configuration classes

It may be preferable to bootstrap the Spring container from XML and include
@Configuration classes in an ad-hoc fashion. For example, in a large existing codebase
that uses Spring XML, it will be easier to create @Configuration classes on an
as-needed basis and include them from the existing XML files. Below you’ll find the
options for using @Configuration classes in this kind of "XML-centric" situation.

Remember that @Configuration classes are ultimately just bean definitions in the
container. In this example, we create a @Configuration class named AppConfig and
include it within system-test-config.xml as a <bean/> definition. Because
<context:annotation-config/> is switched on, the container will recognize the
@Configuration annotation and process the @Bean methods declared in AppConfig
properly.

<beans><!-- enable processing of annotations such as @Autowired and @Configuration --><context:annotation-config/><context:property-placeholderlocation="classpath:/com/acme/jdbc.properties"/><beanclass="com.acme.AppConfig"/><beanclass="org.springframework.jdbc.datasource.DriverManagerDataSource"><propertyname="url"value="${jdbc.url}"/><propertyname="username"value="${jdbc.username}"/><propertyname="password"value="${jdbc.password}"/></bean></beans>

In system-test-config.xml above, the AppConfig<bean/> does not declare an id
element. While it would be acceptable to do so, it is unnecessary given that no other
bean will ever refer to it, and it is unlikely that it will be explicitly fetched from
the container by name. Likewise with the DataSource bean - it is only ever autowired
by type, so an explicit bean id is not strictly required.

Because @Configuration is meta-annotated with @Component, @Configuration-annotated
classes are automatically candidates for component scanning. Using the same scenario as
above, we can redefine system-test-config.xml to take advantage of component-scanning.
Note that in this case, we don’t need to explicitly declare
<context:annotation-config/>, because <context:component-scan/> enables the same
functionality.

system-test-config.xml:

<beans><!-- picks up and registers AppConfig as a bean definition --><context:component-scanbase-package="com.acme"/><context:property-placeholderlocation="classpath:/com/acme/jdbc.properties"/><beanclass="org.springframework.jdbc.datasource.DriverManagerDataSource"><propertyname="url"value="${jdbc.url}"/><propertyname="username"value="${jdbc.username}"/><propertyname="password"value="${jdbc.password}"/></bean></beans>

@Configuration class-centric use of XML with @ImportResource

In applications where @Configuration classes are the primary mechanism for configuring
the container, it will still likely be necessary to use at least some XML. In these
scenarios, simply use @ImportResource and define only as much XML as is needed. Doing
so achieves a "Java-centric" approach to configuring the container and keeps XML to a
bare minimum.

7.13 Environment abstraction

The Environment
is an abstraction integrated in the container that models two key
aspects of the application environment: profiles
and properties.

A profile is a named, logical group of bean definitions to be registered with the
container only if the given profile is active. Beans may be assigned to a profile
whether defined in XML or via annotations. The role of the Environment object with
relation to profiles is in determining which profiles (if any) are currently active,
and which profiles (if any) should be active by default.

Properties play an important role in almost all applications, and may originate from
a variety of sources: properties files, JVM system properties, system environment
variables, JNDI, servlet context parameters, ad-hoc Properties objects, Maps, and so
on. The role of the Environment object with relation to properties is to provide the
user with a convenient service interface for configuring property sources and resolving
properties from them.

7.13.1 Bean definition profiles

Bean definition profiles is a mechanism in the core container that allows for
registration of different beans in different environments. The word environment
can mean different things to different users and this feature can help with many
use cases, including:

working against an in-memory datasource in development vs looking up that same
datasource from JNDI when in QA or production

registering monitoring infrastructure only when deploying an application into a
performance environment

Let’s now consider how this application will be deployed into a QA or production
environment, assuming that the datasource for the application will be registered
with the production application server’s JNDI directory. Our dataSource bean
now looks like this:

The problem is how to switch between using these two variations based on the
current environment. Over time, Spring users have devised a number of ways to
get this done, usually relying on a combination of system environment variables
and XML <import/> statements containing ${placeholder} tokens that resolve
to the correct configuration file path depending on the value of an environment
variable. Bean definition profiles is a core container feature that provides a
solution to this problem.

If we generalize the example use case above of environment-specific bean
definitions, we end up with the need to register certain bean definitions in
certain contexts, while not in others. You could say that you want to register a
certain profile of bean definitions in situation A, and a different profile in
situation B. Let’s first see how we can update our configuration to reflect
this need.

@Profile

The @Profile
annotation allows you to indicate that a component is eligible for registration
when one or more specified profiles are active. Using our example above, we
can rewrite the dataSource configuration as follows:

As mentioned before, with @Bean methods, you will typically choose to use programmatic
JNDI lookups: either using Spring’s JndiTemplate/JndiLocatorDelegate helpers or the
straight JNDI InitialContext usage shown above, but not the JndiObjectFactoryBean
variant which would force you to declare the return type as the FactoryBean type.

@Profile can be used as a meta-annotation for the purpose
of creating a custom composed annotation. The following example defines a custom
@Production annotation that can be used as a drop-in replacement for
@Profile("production"):

If a @Configuration class is marked with @Profile, all of the @Bean methods and
@Import annotations associated with that class will be bypassed unless one or more of
the specified profiles are active. If a @Component or @Configuration class is marked
with @Profile({"p1", "p2"}), that class will not be registered/processed unless
profiles 'p1' and/or 'p2' have been activated. If a given profile is prefixed with the
NOT operator (!), the annotated element will be registered if the profile is not
active. For example, given @Profile({"p1", "!p2"}), registration will occur if profile
'p1' is active or if profile 'p2' is not active.

@Profile can also be declared at the method level to include only one particular bean
of a configuration class, e.g. for alternative variants of a particular bean:

With @Profile on @Bean methods, a special scenario may apply: In the case of
overloaded @Bean methods of the same Java method name (analogous to constructor
overloading), an @Profile condition needs to be consistently declared on all
overloaded methods. If the conditions are inconsistent, only the condition on the
first declaration among the overloaded methods will matter. @Profile can therefore
not be used to select an overloaded method with a particular argument signature over
another; resolution between all factory methods for the same bean follows Spring’s
constructor resolution algorithm at creation time.

If you would like to define alternative beans with different profile conditions,
use distinct Java method names pointing to the same bean name via the @Bean name
attribute, as indicated in the example above. If the argument signatures are all
the same (e.g. all of the variants have no-arg factory methods), this is the only
way to represent such an arrangement in a valid Java class in the first place
(since there can only be one method of a particular name and argument signature).

XML bean definition profiles

The XML counterpart is the profile attribute of the <beans> element. Our sample
configuration above can be rewritten in two XML files as follows:

The spring-bean.xsd has been constrained to allow such elements only as the
last ones in the file. This should help provide flexibility without incurring
clutter in the XML files.

Activating a profile

Now that we have updated our configuration, we still need to instruct Spring which
profile is active. If we started our sample application right now, we would see
a NoSuchBeanDefinitionException thrown, because the container could not find
the Spring bean named dataSource.

Activating a profile can be done in several ways, but the most straightforward is to do
it programmatically against the Environment API which is available via an
ApplicationContext:

Note that profiles are not an "either-or" proposition; it is possible to activate multiple
profiles at once. Programmatically, simply provide multiple profile names to the
setActiveProfiles() method, which accepts String…​ varargs:

ctx.getEnvironment().setActiveProfiles("profile1", "profile2");

Declaratively, spring.profiles.active may accept a comma-separated list of profile names:

-Dspring.profiles.active="profile1,profile2"

Default profile

The default profile represents the profile that is enabled by default. Consider the
following:

If no profile is active, the dataSource above will be created; this can be
seen as a way to provide a default definition for one or more beans. If any
profile is enabled, the default profile will not apply.

The name of the default profile can be changed using setDefaultProfiles() on
the Environment or declaratively using the spring.profiles.default property.

In the snippet above, we see a high-level way of asking Spring whether the foo property is
defined for the current environment. To answer this question, the Environment object performs
a search over a set of PropertySource
objects. A PropertySource is a simple abstraction over any source of key-value pairs, and
Spring’s StandardEnvironment
is configured with two PropertySource objects — one representing the set of JVM system properties
(a laSystem.getProperties()) and one representing the set of system environment variables
(a laSystem.getenv()).

Note

These default property sources are present for StandardEnvironment, for use in standalone
applications. StandardServletEnvironment
is populated with additional default property sources including servlet config and servlet
context parameters. StandardPortletEnvironment
similarly has access to portlet config and portlet context parameters as property sources.
Both can optionally enable a JndiPropertySource.
See the javadocs for details.

Concretely, when using the StandardEnvironment, the call to env.containsProperty("foo")
will return true if a foo system property or foo environment variable is present at
runtime.

Tip

The search performed is hierarchical. By default, system properties have precedence over
environment variables, so if the foo property happens to be set in both places during
a call to env.getProperty("foo"), the system property value will 'win' and be returned
preferentially over the environment variable. Note that property values will not get merged
but rather completely overridden by a preceding entry.

For a common StandardServletEnvironment, the full hierarchy looks as follows, with the
highest-precedence entries at the top:

Most importantly, the entire mechanism is configurable. Perhaps you have a custom source
of properties that you’d like to integrate into this search. No problem — simply implement
and instantiate your own PropertySource and add it to the set of PropertySources for the
current Environment:

In the code above, MyPropertySource has been added with highest precedence in the
search. If it contains a foo property, it will be detected and returned ahead of
any foo property in any other PropertySource. The
MutablePropertySources
API exposes a number of methods that allow for precise manipulation of the set of
property sources.

7.13.3 @PropertySource

The @PropertySource
annotation provides a convenient and declarative mechanism for adding a PropertySource
to Spring’s Environment.

Given a file "app.properties" containing the key/value pair testbean.name=myTestBean,
the following @Configuration class uses @PropertySource in such a way that
a call to testBean.getName() will return "myTestBean".

Assuming that "my.placeholder" is present in one of the property sources already
registered, e.g. system properties or environment variables, the placeholder will
be resolved to the corresponding value. If not, then "default/path" will be used
as a default. If no default is specified and a property cannot be resolved, an
IllegalArgumentException will be thrown.

7.13.4 Placeholder resolution in statements

Historically, the value of placeholders in elements could be resolved only against
JVM system properties or environment variables. No longer is this the case. Because
the Environment abstraction is integrated throughout the container, it’s easy to
route resolution of placeholders through it. This means that you may configure the
resolution process in any way you like: change the precedence of searching through
system properties and environment variables, or remove them entirely; add your
own property sources to the mix as appropriate.

Concretely, the following statement works regardless of where the customer
property is defined, as long as it is available in the Environment:

7.14 Registering a LoadTimeWeaver

The LoadTimeWeaver is used by Spring to dynamically transform classes as they are
loaded into the Java virtual machine (JVM).

To enable load-time weaving add the @EnableLoadTimeWeaving to one of your
@Configuration classes:

@Configuration@EnableLoadTimeWeavingpublicclass AppConfig {
}

Alternatively for XML configuration use the context:load-time-weaver element:

<beans><context:load-time-weaver/></beans>

Once configured for the ApplicationContext. Any bean within that ApplicationContext
may implement LoadTimeWeaverAware, thereby receiving a reference to the load-time
weaver instance. This is particularly useful in combination with Spring’s JPA
support where load-time weaving may be necessary for JPA class transformation. Consult
the LocalContainerEntityManagerFactoryBean javadocs for more detail. For more on
AspectJ load-time weaving, see Section 11.8.4, “Load-time weaving with AspectJ in the Spring Framework”.

7.15 Additional Capabilities of the ApplicationContext

As was discussed in the chapter introduction, the org.springframework.beans.factory
package provides basic functionality for managing and manipulating beans, including in a
programmatic way. The org.springframework.context package adds the
ApplicationContext
interface, which extends the BeanFactory interface, in addition to extending other
interfaces to provide additional functionality in a more application
framework-oriented style. Many people use the ApplicationContext in a completely
declarative fashion, not even creating it programmatically, but instead relying on
support classes such as ContextLoader to automatically instantiate an
ApplicationContext as part of the normal startup process of a Java EE web application.

To enhance BeanFactory functionality in a more framework-oriented style the context
package also provides the following functionality:

Access to messages in i18n-style, through the MessageSource interface.

Access to resources, such as URLs and files, through the ResourceLoader interface.

Event publication to namely beans implementing the ApplicationListener interface,
through the use of the ApplicationEventPublisher interface.

Loading of multiple (hierarchical) contexts, allowing each to be focused on one
particular layer, such as the web layer of an application, through the
HierarchicalBeanFactory interface.

7.15.1 Internationalization using MessageSource

The ApplicationContext interface extends an interface called MessageSource, and
therefore provides internationalization (i18n) functionality. Spring also provides the
interface HierarchicalMessageSource, which can resolve messages hierarchically.
Together these interfaces provide the foundation upon which Spring effects message
resolution. The methods defined on these interfaces include:

String getMessage(String code, Object[] args, String default, Locale loc): The basic
method used to retrieve a message from the MessageSource. When no message is found
for the specified locale, the default message is used. Any arguments passed in become
replacement values, using the MessageFormat functionality provided by the standard
library.

String getMessage(String code, Object[] args, Locale loc): Essentially the same as
the previous method, but with one difference: no default message can be specified; if
the message cannot be found, a NoSuchMessageException is thrown.

String getMessage(MessageSourceResolvable resolvable, Locale locale): All properties
used in the preceding methods are also wrapped in a class named
MessageSourceResolvable, which you can use with this method.

When an ApplicationContext is loaded, it automatically searches for a MessageSource
bean defined in the context. The bean must have the name messageSource. If such a bean
is found, all calls to the preceding methods are delegated to the message source. If no
message source is found, the ApplicationContext attempts to find a parent containing a
bean with the same name. If it does, it uses that bean as the MessageSource. If the
ApplicationContext cannot find any source for messages, an empty
DelegatingMessageSource is instantiated in order to be able to accept calls to the
methods defined above.

Spring provides two MessageSource implementations, ResourceBundleMessageSource and
StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested
messaging. The StaticMessageSource is rarely used but provides programmatic ways to
add messages to the source. The ResourceBundleMessageSource is shown in the following
example:

In the example it is assumed you have three resource bundles defined in your classpath
called format, exceptions and windows. Any request to resolve a message will be
handled in the JDK standard way of resolving messages through ResourceBundles. For the
purposes of the example, assume the contents of two of the above resource bundle files
are…​

A program to execute the MessageSource functionality is shown in the next example.
Remember that all ApplicationContext implementations are also MessageSource
implementations and so can be cast to the MessageSource interface.

So to summarize, the MessageSource is defined in a file called beans.xml, which
exists at the root of your classpath. The messageSource bean definition refers to a
number of resource bundles through its basenames property. The three files that are
passed in the list to the basenames property exist as files at the root of your
classpath and are called format.properties, exceptions.properties, and
windows.properties respectively.

The next example shows arguments passed to the message lookup; these arguments will be
converted into Strings and inserted into placeholders in the lookup message.

<beans><!-- this MessageSource is being used in a web application --><beanid="messageSource"class="org.springframework.context.support.ResourceBundleMessageSource"><propertyname="basename"value="exceptions"/></bean><!-- lets inject the above MessageSource into this POJO --><beanid="example"class="com.foo.Example"><propertyname="messages"ref="messageSource"/></bean></beans>

The resulting output from the invocation of the execute() method will be…​

The userDao argument is required.

With regard to internationalization (i18n), Spring’s various MessageSource
implementations follow the same locale resolution and fallback rules as the standard JDK
ResourceBundle. In short, and continuing with the example messageSource defined
previously, if you want to resolve messages against the British (en-GB) locale, you
would create files called format_en_GB.properties, exceptions_en_GB.properties, and
windows_en_GB.properties respectively.

Typically, locale resolution is managed by the surrounding environment of the
application. In this example, the locale against which (British) messages will be
resolved is specified manually.

# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.

You can also use the MessageSourceAware interface to acquire a reference to any
MessageSource that has been defined. Any bean that is defined in an
ApplicationContext that implements the MessageSourceAware interface is injected with
the application context’s MessageSource when the bean is created and configured.

Note

As an alternative to ResourceBundleMessageSource, Spring provides a
ReloadableResourceBundleMessageSource class. This variant supports the same bundle
file format but is more flexible than the standard JDK based
ResourceBundleMessageSource implementation. In particular, it allows for reading
files from any Spring resource location (not just from the classpath) and supports hot
reloading of bundle property files (while efficiently caching them in between). Check
out the ReloadableResourceBundleMessageSource javadocs for details.

7.15.2 Standard and Custom Events

Event handling in the ApplicationContext is provided through the ApplicationEvent
class and ApplicationListener interface. If a bean that implements the
ApplicationListener interface is deployed into the context, every time an
ApplicationEvent gets published to the ApplicationContext, that bean is notified.
Essentially, this is the standard Observer design pattern.

Tip

As of Spring 4.2, the event infrastructure has been significantly improved and offer
an annotation-based model as well as the
ability to publish any arbitrary event, that is an object that does not necessarily
extend from ApplicationEvent. When such an object is published we wrap it in an
event for you.

Spring provides the following standard events:

Table 7.7. Built-in Events

Event

Explanation

ContextRefreshedEvent

Published when the ApplicationContext is initialized or refreshed, for example,
using the refresh() method on the ConfigurableApplicationContext interface.
"Initialized" here means that all beans are loaded, post-processor beans are detected
and activated, singletons are pre-instantiated, and the ApplicationContext object is
ready for use. As long as the context has not been closed, a refresh can be triggered
multiple times, provided that the chosen ApplicationContext actually supports such
"hot" refreshes. For example, XmlWebApplicationContext supports hot refreshes, but
GenericApplicationContext does not.

ContextStartedEvent

Published when the ApplicationContext is started, using the start() method on the
ConfigurableApplicationContext interface. "Started" here means that all Lifecycle
beans receive an explicit start signal. Typically this signal is used to restart beans
after an explicit stop, but it may also be used to start components that have not been
configured for autostart , for example, components that have not already started on
initialization.

ContextStoppedEvent

Published when the ApplicationContext is stopped, using the stop() method on the
ConfigurableApplicationContext interface. "Stopped" here means that all Lifecycle
beans receive an explicit stop signal. A stopped context may be restarted through a
start() call.

ContextClosedEvent

Published when the ApplicationContext is closed, using the close() method on the
ConfigurableApplicationContext interface. "Closed" here means that all singleton
beans are destroyed. A closed context reaches its end of life; it cannot be refreshed
or restarted.

RequestHandledEvent

A web-specific event telling all beans that an HTTP request has been serviced. This
event is published after the request is complete. This event is only applicable to
web applications using Spring’s DispatcherServlet.

You can also create and publish your own custom events. This example demonstrates a
simple class that extends Spring’s ApplicationEvent base class:

To publish a custom ApplicationEvent, call the publishEvent() method on an
ApplicationEventPublisher. Typically this is done by creating a class that implements
ApplicationEventPublisherAware and registering it as a Spring bean. The following
example demonstrates such a class:

At configuration time, the Spring container will detect that EmailService implements
ApplicationEventPublisherAware and will automatically call
setApplicationEventPublisher(). In reality, the parameter passed in will be the Spring
container itself; you’re simply interacting with the application context via its
ApplicationEventPublisher interface.

To receive the custom ApplicationEvent, create a class that implements
ApplicationListener and register it as a Spring bean. The following example
demonstrates such a class:

Notice that ApplicationListener is generically parameterized with the type of your
custom event, BlackListEvent. This means that the onApplicationEvent() method can
remain type-safe, avoiding any need for downcasting. You may register as many event
listeners as you wish, but note that by default event listeners receive events
synchronously. This means the publishEvent() method blocks until all listeners have
finished processing the event. One advantage of this synchronous and single-threaded
approach is that when a listener receives an event, it operates inside the transaction
context of the publisher if a transaction context is available. If another strategy for
event publication becomes necessary, refer to the javadoc for Spring’s
ApplicationEventMulticaster interface.

The following example shows the bean definitions used to register and configure each of
the classes above:

Putting it all together, when the sendEmail() method of the emailService bean is
called, if there are any emails that should be blacklisted, a custom event of type
BlackListEvent is published. The blackListNotifier bean is registered as an
ApplicationListener and thus receives the BlackListEvent, at which point it can
notify appropriate parties.

Note

Spring’s eventing mechanism is designed for simple communication between Spring beans
within the same application context. However, for more sophisticated enterprise
integration needs, the separately-maintained
Spring Integration project provides
complete support for building lightweight,
pattern-oriented, event-driven
architectures that build upon the well-known Spring programming model.

Annotation-based Event Listeners

As of Spring 4.2, an event listener can be registered on any public method of a managed
bean via the EventListener annotation. The BlackListNotifier can be rewritten as
follows:

As you can see above, the method signature once again declares the event type it listens to,
but this time with a flexible name and without implementing a specific listener interface.
The event type can also be narrowed through generics as long as the actual event type
resolves your generic parameter in its implementation hierarchy.

If your method should listen to several events or if you want to define it with no
parameter at all, the event type(s) can also be specified on the annotation itself:

It is also possible to add additional runtime filtering via the condition attribute of the
annotation that defines a SpEL expression that should match to actually invoke
the method for a particular event.

For instance, our notifier can be rewritten to be only invoked if the test attribute of the
event is equal to foo:

Each SpEL expression evaluates again a dedicated context. The next table lists the items made
available to the context so one can use them for conditional event processing:

Table 7.8. Event SpEL available metadata

Name

Location

Description

Example

Event

root object

The actual ApplicationEvent

#root.event

Arguments array

root object

The arguments (as array) used for invoking the target

#root.args[0]

Argument name

evaluation context

Name of any of the method arguments. If for some reason the names are not available
(e.g. no debug information), the argument names are also available under the #a<#arg>
where #arg stands for the argument index (starting from 0).

#blEvent or #a0 (one can also use #p0 or #p<#arg> notation as an alias).

Note that #root.event allows you to access to the underlying event, even if your method
signature actually refers to an arbitrary object that was published.

If you need to publish an event as the result of processing another, just change the
method signature to return the event that should be published, something like:

Generic Events

You may also use generics to further define the structure of your event. Consider an
EntityCreatedEvent<T> where T is the type of the actual entity that got created. You
can create the following listener definition to only receive EntityCreatedEvent for a
Person:

Due to type erasure, this will only work if the event that is fired resolves the generic
parameter(s) on which the event listener filters on (that is something like
class PersonCreatedEvent extends EntityCreatedEvent<Person> { …​ }).

In certain circumstances, this may become quite tedious if all events follow the same
structure (as it should be the case for the event above). In such a case, you can
implement ResolvableTypeProvider to guide the framework beyond what the runtime
environment provides:

This works not only for ApplicationEvent but any arbitrary object that you’d send as
an event.

7.15.3 Convenient access to low-level resources

For optimal usage and understanding of application contexts, users should generally
familiarize themselves with Spring’s Resource abstraction, as described in the chapter
Chapter 8, Resources.

An application context is a ResourceLoader, which can be used to load Resources. A
Resource is essentially a more feature rich version of the JDK class java.net.URL,
in fact, the implementations of the Resource wrap an instance of java.net.URL where
appropriate. A Resource can obtain low-level resources from almost any location in a
transparent fashion, including from the classpath, a filesystem location, anywhere
describable with a standard URL, and some other variations. If the resource location
string is a simple path without any special prefixes, where those resources come from is
specific and appropriate to the actual application context type.

You can configure a bean deployed into the application context to implement the special
callback interface, ResourceLoaderAware, to be automatically called back at
initialization time with the application context itself passed in as the
ResourceLoader. You can also expose properties of type Resource, to be used to
access static resources; they will be injected into it like any other properties. You
can specify those Resource properties as simple String paths, and rely on a special
JavaBean PropertyEditor that is automatically registered by the context, to convert
those text strings to actual Resource objects when the bean is deployed.

The location path or paths supplied to an ApplicationContext constructor are actually
resource strings, and in simple form are treated appropriately to the specific context
implementation. ClassPathXmlApplicationContext treats a simple location path as a
classpath location. You can also use location paths (resource strings) with special
prefixes to force loading of definitions from the classpath or a URL, regardless of the
actual context type.

You can create ApplicationContext instances declaratively by using, for example, a
ContextLoader. Of course you can also create ApplicationContext instances
programmatically by using one of the ApplicationContext implementations.

You can register an ApplicationContext using the ContextLoaderListener as follows:

The listener inspects the contextConfigLocation parameter. If the parameter does not
exist, the listener uses /WEB-INF/applicationContext.xml as a default. When the
parameter does exist, the listener separates the String by using predefined
delimiters (comma, semicolon and whitespace) and uses the values as locations where
application contexts will be searched. Ant-style path patterns are supported as well.
Examples are /WEB-INF/*Context.xml for all files with names ending with "Context.xml",
residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml, for all such files
in any subdirectory of "WEB-INF".

7.15.5 Deploying a Spring ApplicationContext as a Java EE RAR file

It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the
context and all of its required bean classes and library JARs in a Java EE RAR deployment
unit. This is the equivalent of bootstrapping a standalone ApplicationContext, just hosted
in Java EE environment, being able to access the Java EE servers facilities. RAR deployment
is more natural alternative to scenario of deploying a headless WAR file, in effect, a WAR
file without any HTTP entry points that is used only for bootstrapping a Spring
ApplicationContext in a Java EE environment.

RAR deployment is ideal for application contexts that do not need HTTP entry points but
rather consist only of message endpoints and scheduled jobs. Beans in such a context can
use application server resources such as the JTA transaction manager and JNDI-bound JDBC
DataSources and JMS ConnectionFactory instances, and may also register with the
platform’s JMX server - all through Spring’s standard transaction management and JNDI
and JMX support facilities. Application components can also interact with the
application server’s JCA WorkManager through Spring’s TaskExecutor abstraction.

For a simple deployment of a Spring ApplicationContext as a Java EE RAR file: package
all application classes into a RAR file, which is a standard JAR file with a different
file extension. Add all required library JARs into the root of the RAR archive. Add a
"META-INF/ra.xml" deployment descriptor (as shown in SpringContextResourceAdapters
javadoc) and the corresponding Spring XML bean definition file(s) (typically
"META-INF/applicationContext.xml"), and drop the resulting RAR file into your
application server’s deployment directory.

Note

Such RAR deployment units are usually self-contained; they do not expose components to
the outside world, not even to other modules of the same application. Interaction with a
RAR-based ApplicationContext usually occurs through JMS destinations that it shares with
other modules. A RAR-based ApplicationContext may also, for example, schedule some jobs,
reacting to new files in the file system (or the like). If it needs to allow synchronous
access from the outside, it could for example export RMI endpoints, which of course may
be used by other application modules on the same machine.

7.16 The BeanFactory

The BeanFactory provides the underlying basis for Spring’s IoC functionality but it is
only used directly in integration with other third-party frameworks and is now largely
historical in nature for most users of Spring. The BeanFactory and related interfaces,
such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in
Spring for the purposes of backward compatibility with the large number of third-party
frameworks that integrate with Spring. Often third-party components that can not use
more modern equivalents such as @PostConstruct or @PreDestroy in order to remain
compatible with JDK 1.4 or to avoid a dependency on JSR-250.

This section provides additional background into the differences between the
BeanFactory and ApplicationContext and how one might access the IoC container
directly through a classic singleton lookup.

7.16.1 BeanFactory or ApplicationContext?

Use an ApplicationContext unless you have a good reason for not doing so.

Because the ApplicationContext includes all functionality of the BeanFactory, it is
generally recommended over the BeanFactory, except for a few situations such as in
embedded applications running on resource-constrained devices where memory consumption
might be critical and a few extra kilobytes might make a difference. However, for
most typical enterprise applications and systems, the ApplicationContext is what you
will want to use. Spring makes heavy use of the BeanPostProcessor extension point (to effect proxying and so on). If you use only a
plain BeanFactory, a fair amount of support such as transactions and AOP will not take
effect, at least not without some extra steps on your part. This situation could be
confusing because nothing is actually wrong with the configuration.

The following table lists features provided by the BeanFactory and
ApplicationContext interfaces and implementations.

Table 7.9. Feature Matrix

Feature

BeanFactory

ApplicationContext

Bean instantiation/wiring

Yes

Yes

Automatic BeanPostProcessor registration

No

Yes

Automatic BeanFactoryPostProcessor registration

No

Yes

Convenient MessageSource access (for i18n)

No

Yes

ApplicationEvent publication

No

Yes

To explicitly register a bean post-processor with a BeanFactory implementation,
you need to write code like this:

DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
// populate the factory with bean definitions// now register any needed BeanPostProcessor instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);
// now start using the factory

To explicitly register a BeanFactoryPostProcessor when using a BeanFactory
implementation, you must write code like this:

In both cases, the explicit registration step is inconvenient, which is one reason why
the various ApplicationContext implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially when
using BeanFactoryPostProcessors and BeanPostProcessors. These mechanisms implement
important functionality such as property placeholder replacement and AOP.

7.16.2 Glue code and the evil singleton

It is best to write most application code in a dependency-injection (DI) style, where
that code is served out of a Spring IoC container, has its own dependencies supplied by
the container when it is created, and is completely unaware of the container. However,
for the small glue layers of code that are sometimes needed to tie other code together,
you sometimes need a singleton (or quasi-singleton) style access to a Spring IoC
container. For example, third-party code may try to construct new objects directly (
Class.forName() style), without the ability to get these objects out of a Spring IoC
container.If the object constructed by the third-party code is a small stub or proxy,
which then uses a singleton style access to a Spring IoC container to get a real object
to delegate to, then inversion of control has still been achieved for the majority of
the code (the object coming out of the container). Thus most code is still unaware of
the container or how it is accessed, and remains decoupled from other code, with all
ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java
implementation object, retrieved from a Spring IoC container. While the Spring IoC
container itself ideally does not have to be a singleton, it may be unrealistic in terms
of memory usage or initialization times (when using beans in the Spring IoC container
such as a Hibernate SessionFactory) for each bean to use its own, non-singleton Spring
IoC container.

Looking up the application context in a service locator style is sometimes the only
option for accessing shared Spring-managed components, such as in an EJB 2.1
environment, or when you want to share a single ApplicationContext as a parent to
WebApplicationContexts across WAR files. In this case you should look into using the
utility class
ContextSingletonBeanFactoryLocator
locator that is described in this
Spring
team blog entry.

8. Resources

8.1 Introduction

Java’s standard java.net.URL class and standard handlers for various URL prefixes
unfortunately are not quite adequate enough for all access to low-level resources. For
example, there is no standardized URL implementation that may be used to access a
resource that needs to be obtained from the classpath, or relative to a
ServletContext. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:), this is generally
quite complicated, and the URL interface still lacks some desirable functionality,
such as a method to check for the existence of the resource being pointed to.

8.2 The Resource interface

Spring’s Resource interface is meant to be a more capable interface for abstracting
access to low-level resources.

getInputStream(): locates and opens the resource, returning an InputStream for
reading from the resource. It is expected that each invocation returns a fresh
InputStream. It is the responsibility of the caller to close the stream.

isOpen(): returns a boolean indicating whether this resource represents a handle
with an open stream. If true, the InputStream cannot be read multiple times, and
must be read once only and then closed to avoid resource leaks. Will be false for
all usual resource implementations, with the exception of InputStreamResource.

getDescription(): returns a description for this resource, to be used for error
output when working with the resource. This is often the fully qualified file name or
the actual URL of the resource.

Other methods allow you to obtain an actual URL or File object representing the
resource (if the underlying implementation is compatible, and supports that
functionality).

The Resource abstraction is used extensively in Spring itself, as an argument type in
many method signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various ApplicationContext implementations), take a
String which in unadorned or simple form is used to create a Resource appropriate to
that context implementation, or via special prefixes on the String path, allow the
caller to specify that a specific Resource implementation must be created and used.

While the Resource interface is used a lot with Spring and by Spring, it’s actually
very useful to use as a general utility class by itself in your own code, for access to
resources, even when your code doesn’t know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this small set of
utility classes, which are serving as a more capable replacement for URL, and can be
considered equivalent to any other library you would use for this purpose.

It is important to note that the Resource abstraction does not replace functionality:
it wraps it where possible. For example, a UrlResource wraps a URL, and uses the
wrapped URL to do its work.

8.3 Built-in Resource implementations

There are a number of Resource implementations that come supplied straight out of the
box in Spring:

8.3.1 UrlResource

The UrlResource wraps a java.net.URL, and may be used to access any object that is
normally accessible via a URL, such as files, an HTTP target, an FTP target, etc. All
URLs have a standardized String representation, such that appropriate standardized
prefixes are used to indicate one URL type from another. This includes file: for
accessing filesystem paths, http: for accessing resources via the HTTP protocol,
ftp: for accessing resources via FTP, etc.

A UrlResource is created by Java code explicitly using the UrlResource constructor,
but will often be created implicitly when you call an API method which takes a String
argument which is meant to represent a path. For the latter case, a JavaBeans
PropertyEditor will ultimately decide which type of Resource to create. If the path
string contains a few well-known (to it, that is) prefixes such as classpath:, it will
create an appropriate specialized Resource for that prefix. However, if it doesn’t
recognize the prefix, it will assume the this is just a standard URL string, and will
create a UrlResource.

8.3.2 ClassPathResource

This class represents a resource which should be obtained from the classpath. This uses
either the thread context class loader, a given class loader, or a given class for
loading resources.

This Resource implementation supports resolution as java.io.File if the class path
resource resides in the file system, but not for classpath resources which reside in a
jar and have not been expanded (by the servlet engine, or whatever the environment is)
to the filesystem. To address this the various Resource implementations always support
resolution as a java.net.URL.

A ClassPathResource is created by Java code explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API method which
takes a String argument which is meant to represent a path. For the latter case, a
JavaBeans PropertyEditor will recognize the special prefix classpath: on the string
path, and create a ClassPathResource in that case.

8.3.3 FileSystemResource

This is a Resource implementation for java.io.File handles. It obviously supports
resolution as a File, and as a URL.

8.3.4 ServletContextResource

This is a Resource implementation for ServletContext resources, interpreting
relative paths within the relevant web application’s root directory.

This always supports stream access and URL access, but only allows java.io.File access
when the web application archive is expanded and the resource is physically on the
filesystem. Whether or not it’s expanded and on the filesystem like this, or accessed
directly from the JAR or somewhere else like a DB (it’s conceivable) is actually
dependent on the Servlet container.

8.3.5 InputStreamResource

A Resource implementation for a given InputStream. This should only be used if no
specific Resource implementation is applicable. In particular, prefer
ByteArrayResource or any of the file-based Resource implementations where possible.

In contrast to other Resource implementations, this is a descriptor for an already
opened resource - therefore returning true from isOpen(). Do not use it if you need
to keep the resource descriptor somewhere, or if you need to read a stream multiple
times.

8.3.6 ByteArrayResource

This is a Resource implementation for a given byte array. It creates a
ByteArrayInputStream for the given byte array.

It’s useful for loading content from any given byte array, without having to resort to a
single-use InputStreamResource.

8.4 The ResourceLoader

The ResourceLoader interface is meant to be implemented by objects that can return
(i.e. load) Resource instances.

All application contexts implement the ResourceLoader interface, and therefore all
application contexts may be used to obtain Resource instances.

When you call getResource() on a specific application context, and the location path
specified doesn’t have a specific prefix, you will get back a Resource type that is
appropriate to that particular application context. For example, assume the following
snippet of code was executed against a ClassPathXmlApplicationContext instance:

What would be returned would be a ClassPathResource; if the same method was executed
against a FileSystemXmlApplicationContext instance, you’d get back a
FileSystemResource. For a WebApplicationContext, you’d get back a
ServletContextResource, and so on.

As such, you can load resources in a fashion appropriate to the particular application
context.

On the other hand, you may also force ClassPathResource to be used, regardless of the
application context type, by specifying the special classpath: prefix:

When a class implements ResourceLoaderAware and is deployed into an application
context (as a Spring-managed bean), it is recognized as ResourceLoaderAware by the
application context. The application context will then invoke the
setResourceLoader(ResourceLoader), supplying itself as the argument (remember, all
application contexts in Spring implement the ResourceLoader interface).

Of course, since an ApplicationContext is a ResourceLoader, the bean could also
implement the ApplicationContextAware interface and use the supplied application
context directly to load resources, but in general, it’s better to use the specialized
ResourceLoader interface if that’s all that’s needed. The code would just be coupled
to the resource loading interface, which can be considered a utility interface, and not
the whole Spring ApplicationContext interface.

As of Spring 2.5, you can rely upon autowiring of the ResourceLoader as an alternative
to implementing the ResourceLoaderAware interface. The "traditional" constructor and
byType autowiring modes (as described in Section 7.4.5, “Autowiring collaborators”) are now capable
of providing a dependency of type ResourceLoader for either a constructor argument or
setter method parameter respectively. For more flexibility (including the ability to
autowire fields and multiple parameter methods), consider using the new annotation-based
autowiring features. In that case, the ResourceLoader will be autowired into a field,
constructor argument, or method parameter that is expecting the ResourceLoader type as
long as the field, constructor, or method in question carries the @Autowired
annotation. For more information, see Section 7.9.2, “@Autowired”.

8.6 Resources as dependencies

If the bean itself is going to determine and supply the resource path through some sort
of dynamic process, it probably makes sense for the bean to use the ResourceLoader
interface to load resources. Consider as an example the loading of a template of some
sort, where the specific resource that is needed depends on the role of the user. If the
resources are static, it makes sense to eliminate the use of the ResourceLoader
interface completely, and just have the bean expose the Resource properties it needs,
and expect that they will be injected into it.

What makes it trivial to then inject these properties, is that all application contexts
register and use a special JavaBeans PropertyEditor which can convert String paths
to Resource objects. So if myBean has a template property of type Resource, it can
be configured with a simple string for that resource, as follows:

Note that the resource path has no prefix, so because the application context itself is
going to be used as the ResourceLoader, the resource itself will be loaded via a
ClassPathResource, FileSystemResource, or ServletContextResource (as appropriate)
depending on the exact type of the context.

If there is a need to force a specific Resource type to be used, then a prefix may be
used. The following two examples show how to force a ClassPathResource and a
UrlResource (the latter being used to access a filesystem file).

8.7 Application contexts and Resource paths

8.7.1 Constructing application contexts

An application context constructor (for a specific application context type) generally
takes a string or array of strings as the location path(s) of the resource(s) such as
XML files that make up the definition of the context.

When such a location path doesn’t have a prefix, the specific Resource type built from
that path and used to load the bean definitions, depends on and is appropriate to the
specific application context. For example, if you create a
ClassPathXmlApplicationContext as follows:

The bean definition will be loaded from a filesystem location, in this case relative to
the current working directory.

Note that the use of the special classpath prefix or a standard URL prefix on the
location path will override the default type of Resource created to load the
definition. So this FileSystemXmlApplicationContext…​

will actually load its bean definitions from the classpath. However, it is still a
FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any
unprefixed paths will still be treated as filesystem paths.

Constructing ClassPathXmlApplicationContext instances - shortcuts

The ClassPathXmlApplicationContext exposes a number of constructors to enable
convenient instantiation. The basic idea is that one supplies merely a string array
containing just the filenames of the XML files themselves (without the leading path
information), and one also supplies a Class; the ClassPathXmlApplicationContext
will derive the path information from the supplied class.

An example will hopefully make this clear. Consider a directory layout that looks like
this:

com/
foo/
services.xml
daos.xml
MessengerService.class

A ClassPathXmlApplicationContext instance composed of the beans defined in the
'services.xml' and 'daos.xml' could be instantiated like so…​

Please do consult the ClassPathXmlApplicationContext javadocs for details
on the various constructors.

8.7.2 Wildcards in application context constructor resource paths

The resource paths in application context constructor values may be a simple path (as
shown above) which has a one-to-one mapping to a target Resource, or alternately may
contain the special "classpath*:" prefix and/or internal Ant-style regular expressions
(matched using Spring’s PathMatcher utility). Both of the latter are effectively
wildcards

One use for this mechanism is when doing component-style application assembly. All
components can 'publish' context definition fragments to a well-known location path, and
when the final application context is created using the same path prefixed via
classpath*:, all component fragments will be picked up automatically.

Note that this wildcarding is specific to use of resource paths in application context
constructors (or when using the PathMatcher utility class hierarchy directly), and is
resolved at construction time. It has nothing to do with the Resource type itself.
It’s not possible to use the classpath*: prefix to construct an actual Resource, as
a resource points to just one resource at a time.

The resolver follows a more complex but defined procedure to try to resolve the
wildcard. It produces a Resource for the path up to the last non-wildcard segment and
obtains a URL from it. If this URL is not a jar: URL or container-specific variant
(e.g. zip: in WebLogic, wsjar in WebSphere, etc.), then a java.io.File is
obtained from it and used to resolve the wildcard by traversing the filesystem. In the
case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or
manually parses the jar URL and then traverses the contents of the jar file to resolve
the wildcards.

Implications on portability

If the specified path is already a file URL (either explicitly, or implicitly because
the base ResourceLoader is a filesystem one, then wildcarding is guaranteed to work in
a completely portable fashion.

If the specified path is a classpath location, then the resolver must obtain the last
non-wildcard path segment URL via a Classloader.getResource() call. Since this is just
a node of the path (not the file at the end) it is actually undefined (in the
ClassLoader javadocs) exactly what sort of a URL is returned in this case. In
practice, it is always a java.io.File representing the directory, where the classpath
resource resolves to a filesystem location, or a jar URL of some sort, where the
classpath resource resolves to a jar location. Still, there is a portability concern on
this operation.

If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to
get a java.net.JarURLConnection from it, or manually parse the jar URL, to be able to
walk the contents of the jar, and resolve the wildcard. This will work in most
environments, but will fail in others, and it is strongly recommended that the wildcard
resolution of resources coming from jars be thoroughly tested in your specific
environment before you rely on it.

The Classpath*: portability classpath*: prefix

When constructing an XML-based application context, a location string may use the
special classpath*: prefix:

This special prefix specifies that all classpath resources that match the given name
must be obtained (internally, this essentially happens via a
ClassLoader.getResources(…​) call), and then merged to form the final application
context definition.

Note

The wildcard classpath relies on the getResources() method of the underlying
classloader. As most application servers nowadays supply their own classloader
implementation, the behavior might differ especially when dealing with jar files. A
simple test to check if classpath* works is to use the classloader to load a file from
within a jar on the classpath:
getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try this test with
files that have the same name but are placed inside two different locations. In case an
inappropriate result is returned, check the application server documentation for
settings that might affect the classloader behavior.

The classpath*: prefix can also be combined with a PathMatcher pattern in the
rest of the location path, for example classpath*:META-INF/*-beans.xml. In this
case, the resolution strategy is fairly simple: a ClassLoader.getResources() call is
used on the last non-wildcard path segment to get all the matching resources in the
class loader hierarchy, and then off each resource the same PathMatcher resolution
strategy described above is used for the wildcard subpath.

Other notes relating to wildcards

Please note that classpath*: when combined with Ant-style patterns will only work
reliably with at least one root directory before the pattern starts, unless the actual
target files reside in the file system. This means that a pattern like
classpath*:*.xml will not retrieve files from the root of jar files but rather only
from the root of expanded directories. This originates from a limitation in the JDK’s
ClassLoader.getResources() method which only returns file system locations for a
passed-in empty string (indicating potential roots to search).

Ant-style patterns with classpath: resources are not guaranteed to find matching
resources if the root package to search is available in multiple class path locations.
This is because a resource such as

com/mycompany/package1/service-context.xml

may be in only one location, but when a path such as

classpath:com/mycompany/**/service-context.xml

is used to try to resolve it, the resolver will work off the (first) URL returned by
getResource("com/mycompany");. If this base package node exists in multiple
classloader locations, the actual end resource may not be underneath. Therefore,
preferably, use " `classpath*:`" with the same Ant-style pattern in such a case, which
will search all class path locations that contain the root package.

8.7.3 FileSystemResource caveats

A FileSystemResource that is not attached to a FileSystemApplicationContext (that
is, a FileSystemApplicationContext is not the actual ResourceLoader) will treat
absolute vs. relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root of the
filesystem.

For backwards compatibility (historical) reasons however, this changes when the
FileSystemApplicationContext is the ResourceLoader. The
FileSystemApplicationContext simply forces all attached FileSystemResource instances
to treat all location paths as relative, whether they start with a leading slash or not.
In practice, this means the following are equivalent:

In practice, if true absolute filesystem paths are needed, it is better to forgo the use
of absolute paths with FileSystemResource / FileSystemXmlApplicationContext, and
just force the use of a UrlResource, by using the file: URL prefix.

An application can choose to enable Bean Validation once globally, as described in
Section 9.8, “Spring Validation”, and use it exclusively for all validation needs.

An application can also register additional Spring Validator instances per
DataBinder instance, as described in Section 9.8.3, “Configuring a DataBinder”. This may be useful for
plugging in validation logic without the use of annotations.

There are pros and cons for considering validation as business logic, and Spring offers
a design for validation (and data binding) that does not exclude either one of them.
Specifically validation should not be tied to the web tier, should be easy to localize
and it should be possible to plug in any validator available. Considering the above,
Spring has come up with a Validator interface that is both basic and eminently usable
in every layer of an application.

Data binding is useful for allowing user input to be dynamically bound to the domain
model of an application (or whatever objects you use to process user input). Spring
provides the so-called DataBinder to do exactly that. The Validator and the
DataBinder make up the validation package, which is primarily used in but not
limited to the MVC framework.

The BeanWrapper is a fundamental concept in the Spring Framework and is used in a lot
of places. However, you probably will not have the need to use the BeanWrapper
directly. Because this is reference documentation however, we felt that some explanation
might be in order. We will explain the BeanWrapper in this chapter since, if you were
going to use it at all, you would most likely do so when trying to bind data to objects.

Spring’s DataBinder and the lower-level BeanWrapper both use PropertyEditors to parse
and format property values. The PropertyEditor concept is part of the JavaBeans
specification, and is also explained in this chapter. Spring 3 introduces a
"core.convert" package that provides a general type conversion facility, as well as a
higher-level "format" package for formatting UI field values. These new packages may be
used as simpler alternatives to PropertyEditors, and will also be discussed in this
chapter.

9.2 Validation using Spring’s Validator interface

Spring features a Validator interface that you can use to validate objects. The
Validator interface works using an Errors object so that while validating,
validators can report validation failures to the Errors object.

As you can see, the staticrejectIfEmpty(..) method on the ValidationUtils class
is used to reject the 'name' property if it is null or the empty string. Have a look
at the ValidationUtils javadocs to see what functionality it provides besides the
example shown previously.

While it is certainly possible to implement a single Validator class to validate each
of the nested objects in a rich object, it may be better to encapsulate the validation
logic for each nested class of object in its own Validator implementation. A simple
example of a 'rich' object would be a Customer that is composed of two String
properties (a first and second name) and a complex Address object. Address objects
may be used independently of Customer objects, and so a distinct AddressValidator
has been implemented. If you want your CustomerValidator to reuse the logic contained
within the AddressValidator class without resorting to copy-and-paste, you can
dependency-inject or instantiate an AddressValidator within your CustomerValidator,
and use it like so:

Validation errors are reported to the Errors object passed to the validator. In case
of Spring Web MVC you can use <spring:bind/> tag to inspect the error messages, but of
course you can also inspect the errors object yourself. More information about the
methods it offers can be found in the javadocs.

9.3 Resolving codes to error messages

We’ve talked about databinding and validation. Outputting messages corresponding to
validation errors is the last thing we need to discuss. In the example we’ve shown
above, we rejected the name and the age field. If we’re going to output the error
messages by using a MessageSource, we will do so using the error code we’ve given when
rejecting the field ('name' and 'age' in this case). When you call (either directly, or
indirectly, using for example the ValidationUtils class) rejectValue or one of the
other reject methods from the Errors interface, the underlying implementation will
not only register the code you’ve passed in, but also a number of additional error
codes. What error codes it registers is determined by the MessageCodesResolver that is
used. By default, the DefaultMessageCodesResolver is used, which for example not only
registers a message with the code you gave, but also messages that include the field
name you passed to the reject method. So in case you reject a field using
rejectValue("age", "too.darn.old"), apart from the too.darn.old code, Spring will
also register too.darn.old.age and too.darn.old.age.int (so the first will include
the field name and the second will include the type of the field); this is done as a
convenience to aid developers in targeting error messages and suchlike.

9.4 Bean manipulation and the BeanWrapper

The org.springframework.beans package adheres to the JavaBeans standard provided by
Oracle. A JavaBean is simply a class with a default no-argument constructor, which follows
a naming convention where (by way of an example) a property named bingoMadness would
have a setter method setBingoMadness(..) and a getter method getBingoMadness(). For
more information about JavaBeans and the specification, please refer to Oracle’s website (
javabeans).

One quite important class in the beans package is the BeanWrapper interface and its
corresponding implementation ( BeanWrapperImpl). As quoted from the javadocs, the
BeanWrapper offers functionality to set and get property values (individually or in
bulk), get property descriptors, and to query properties to determine if they are
readable or writable. Also, the BeanWrapper offers support for nested properties,
enabling the setting of properties on sub-properties to an unlimited depth. Then, the
BeanWrapper supports the ability to add standard JavaBeans PropertyChangeListeners
and VetoableChangeListeners, without the need for supporting code in the target class.
Last but not least, the BeanWrapper provides support for the setting of indexed
properties. The BeanWrapper usually isn’t used by application code directly, but by
the DataBinder and the BeanFactory.

The way the BeanWrapper works is partly indicated by its name: it wraps a bean to
perform actions on that bean, like setting and retrieving properties.

9.4.1 Setting and getting basic and nested properties

Setting and getting properties is done using the setPropertyValue(s) and
getPropertyValue(s) methods that both come with a couple of overloaded variants.
They’re all described in more detail in the javadocs Spring comes with. What’s important
to know is that there are a couple of conventions for indicating properties of an
object. A couple of examples:

Table 9.1. Examples of properties

Expression

Explanation

name

Indicates the property name corresponding to the methods getName() or isName()
and setName(..)

account.name

Indicates the nested property name of the property account corresponding e.g. to
the methods getAccount().setName() or getAccount().getName()

account[2]

Indicates the third element of the indexed property account. Indexed properties
can be of type array, list or other naturally ordered collection

account[COMPANYNAME]

Indicates the value of the map entry indexed by the key COMPANYNAME of the Map
property account

Below you’ll find some examples of working with the BeanWrapper to get and set
properties.

(This next section is not vitally important to you if you’re not planning to work with
the BeanWrapper directly. If you’re just using the DataBinder and the BeanFactory
and their out-of-the-box implementation, you should skip ahead to the section about
PropertyEditors.)

The following code snippets show some examples of how to retrieve and manipulate some of
the properties of instantiated Companies and Employees:

BeanWrapper company = new BeanWrapperImpl(new Company());
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.");
// ... can also be done like this:
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);
// ok, let's create the director and tie it to the company:
BeanWrapper jim = new BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());
// retrieving the salary of the managingDirector through the company
Float salary = (Float) company.getPropertyValue("managingDirector.salary");

9.4.2 Built-in PropertyEditor implementations

Spring uses the concept of PropertyEditors to effect the conversion between an
Object and a String. If you think about it, it sometimes might be handy to be able
to represent properties in a different way than the object itself. For example, a Date
can be represented in a human readable way (as the String'2007-14-09'), while
we’re still able to convert the human readable form back to the original date (or even
better: convert any date entered in a human readable form, back to Date objects). This
behavior can be achieved by registering custom editors, of type
java.beans.PropertyEditor. Registering custom editors on a BeanWrapper or
alternately in a specific IoC container as mentioned in the previous chapter, gives it
the knowledge of how to convert properties to the desired type. Read more about
PropertyEditors in the javadocs of the java.beans package provided by Oracle.

A couple of examples where property editing is used in Spring:

setting properties on beans is done using PropertyEditors. When mentioning
java.lang.String as the value of a property of some bean you’re declaring in XML
file, Spring will (if the setter of the corresponding property has a
Class-parameter) use the ClassEditor to try to resolve the parameter to a Class
object.

parsing HTTP request parameters in Spring’s MVC framework is done using all kinds
of PropertyEditors that you can manually bind in all subclasses of the
CommandController.

Spring has a number of built-in PropertyEditors to make life easy. Each of those is
listed below and they are all located in the org.springframework.beans.propertyeditors
package. Most, but not all (as indicated below), are registered by default by
BeanWrapperImpl. Where the property editor is configurable in some fashion, you can of
course still register your own variant to override the default one:

Table 9.2. Built-in PropertyEditors

Class

Explanation

ByteArrayPropertyEditor

Editor for byte arrays. Strings will simply be converted to their corresponding byte
representations. Registered by default by BeanWrapperImpl.

ClassEditor

Parses Strings representing classes to actual classes and the other way around. When a
class is not found, an IllegalArgumentException is thrown. Registered by default by
BeanWrapperImpl.

CustomBooleanEditor

Customizable property editor for Boolean properties. Registered by default by
BeanWrapperImpl, but, can be overridden by registering custom instance of it as
custom editor.

CustomCollectionEditor

Property editor for Collections, converting any source Collection to a given target
Collection type.

CustomDateEditor

Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT
registered by default. Must be user registered as needed with appropriate format.

CustomNumberEditor

Customizable property editor for any Number subclass like Integer, Long, Float,
Double. Registered by default by BeanWrapperImpl, but can be overridden by
registering custom instance of it as a custom editor.

FileEditor

Capable of resolving Strings to java.io.File objects. Registered by default by
BeanWrapperImpl.

InputStreamEditor

One-way property editor, capable of taking a text string and producing (via an
intermediate ResourceEditor and Resource) an InputStream, so InputStream
properties may be directly set as Strings. Note that the default usage will not close
the InputStream for you! Registered by default by BeanWrapperImpl.

LocaleEditor

Capable of resolving Strings to Locale objects and vice versa (the String format is
[country][variant], which is the same thing the toString() method of
Locale provides). Registered by default by BeanWrapperImpl.

PatternEditor

Capable of resolving Strings to java.util.regex.Pattern objects and vice versa.

PropertiesEditor

Capable of converting Strings (formatted using the format as defined in the javadocs
of the java.util.Properties class) to Properties objects. Registered by default
by BeanWrapperImpl.

StringTrimmerEditor

Property editor that trims Strings. Optionally allows transforming an empty string
into a null value. NOT registered by default; must be user registered as needed.

URLEditor

Capable of resolving a String representation of a URL to an actual URL object.
Registered by default by BeanWrapperImpl.

Spring uses the java.beans.PropertyEditorManager to set the search path for property
editors that might be needed. The search path also includes sun.bean.editors, which
includes PropertyEditor implementations for types such as Font, Color, and most of
the primitive types. Note also that the standard JavaBeans infrastructure will
automatically discover PropertyEditor classes (without you having to register them
explicitly) if they are in the same package as the class they handle, and have the same
name as that class, with 'Editor' appended; for example, one could have the following
class and package structure, which would be sufficient for the FooEditor class to be
recognized and used as the PropertyEditor for Foo-typed properties.

com
chank
pop
Foo
FooEditor // the PropertyEditor for the Foo class

Note that you can also use the standard BeanInfo JavaBeans mechanism here as well
(described
in
not-amazing-detail here). Find below an example of using the BeanInfo mechanism for
explicitly registering one or more PropertyEditor instances with the properties of an
associated class.

com
chank
pop
Foo
FooBeanInfo // the BeanInfo for the Foo class

Here is the Java source code for the referenced FooBeanInfo class. This would
associate a CustomNumberEditor with the age property of the Foo class.

Registering additional custom PropertyEditors

When setting bean properties as a string value, a Spring IoC container ultimately uses
standard JavaBeans PropertyEditors to convert these Strings to the complex type of the
property. Spring pre-registers a number of custom PropertyEditors (for example, to
convert a classname expressed as a string into a real Class object). Additionally,
Java’s standard JavaBeans PropertyEditor lookup mechanism allows a PropertyEditor
for a class simply to be named appropriately and placed in the same package as the class
it provides support for, to be found automatically.

If there is a need to register other custom PropertyEditors, there are several
mechanisms available. The most manual approach, which is not normally convenient or
recommended, is to simply use the registerCustomEditor() method of the
ConfigurableBeanFactory interface, assuming you have a BeanFactory reference.
Another, slightly more convenient, mechanism is to use a special bean factory
post-processor called CustomEditorConfigurer. Although bean factory post-processors
can be used with BeanFactory implementations, the CustomEditorConfigurer has a
nested property setup, so it is strongly recommended that it is used with the
ApplicationContext, where it may be deployed in similar fashion to any other bean, and
automatically detected and applied.

Note that all bean factories and application contexts automatically use a number of
built-in property editors, through their use of something called a BeanWrapper to
handle property conversions. The standard property editors that the BeanWrapper
registers are listed in the previous section. Additionally,
ApplicationContexts also override or add an additional number of editors to handle
resource lookups in a manner appropriate to the specific application context type.

Standard JavaBeans PropertyEditor instances are used to convert property values
expressed as strings to the actual complex type of the property.
CustomEditorConfigurer, a bean factory post-processor, may be used to conveniently add
support for additional PropertyEditor instances to an ApplicationContext.

Consider a user class ExoticType, and another class DependsOnExoticType which needs
ExoticType set as a property:

Using PropertyEditorRegistrars

Another mechanism for registering property editors with the Spring container is to
create and use a PropertyEditorRegistrar. This interface is particularly useful when
you need to use the same set of property editors in several different situations: write
a corresponding registrar and reuse that in each case. PropertyEditorRegistrars work
in conjunction with an interface called PropertyEditorRegistry, an interface that is
implemented by the Spring BeanWrapper (and DataBinder). PropertyEditorRegistrars
are particularly convenient when used in conjunction with the CustomEditorConfigurer
(introduced here), which exposes a
property called setPropertyEditorRegistrars(..): PropertyEditorRegistrars added to a
CustomEditorConfigurer in this fashion can easily be shared with DataBinder and
Spring MVC Controllers. Furthermore, it avoids the need for synchronization on custom
editors: a PropertyEditorRegistrar is expected to create fresh PropertyEditor
instances for each bean creation attempt.

Using a PropertyEditorRegistrar is perhaps best illustrated with an example. First
off, you need to create your own PropertyEditorRegistrar implementation:

package com.foo.editors.spring;
publicfinalclass CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {
publicvoid registerCustomEditors(PropertyEditorRegistry registry) {
// it is expected that new PropertyEditor instances are created
registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());
// you could register as many custom property editors as are required here...
}
}

See also the org.springframework.beans.support.ResourceEditorRegistrar for an example
PropertyEditorRegistrar implementation. Notice how in its implementation of the
registerCustomEditors(..) method it creates new instances of each property editor.

Next we configure a CustomEditorConfigurer and inject an instance of our
CustomPropertyEditorRegistrar into it:

Finally, and in a bit of a departure from the focus of this chapter, for those of you
using Spring’s MVC web framework, using PropertyEditorRegistrars in
conjunction with data-binding Controllers (such as SimpleFormController) can be very
convenient. Find below an example of using a PropertyEditorRegistrar in the
implementation of an initBinder(..) method:

This style of PropertyEditor registration can lead to concise code (the implementation
of initBinder(..) is just one line long!), and allows common PropertyEditor
registration code to be encapsulated in a class and then shared amongst as many
Controllers as needed.

9.5 Spring Type Conversion

Spring 3 introduces a core.convert package that provides a general type conversion
system. The system defines an SPI to implement type conversion logic, as well as an API
to execute type conversions at runtime. Within a Spring container, this system can be
used as an alternative to PropertyEditors to convert externalized bean property value
strings to required property types. The public API may also be used anywhere in your
application where type conversion is needed.

9.5.1 Converter SPI

The SPI to implement type conversion logic is simple and strongly typed:

To create your own converter, simply implement the interface above. Parameterize S
as the type you are converting from, and T as the type you are converting to. Such a
converter can also be applied transparently if a collection or array of S needs to be
converted to an array or collection of T, provided that a delegating array/collection
converter has been registered as well (which DefaultConversionService does by default).

For each call to convert(S), the source argument is guaranteed to be NOT null. Your
Converter may throw any unchecked exception if conversion fails; specifically, an
IllegalArgumentException should be thrown to report an invalid source value.
Take care to ensure that your Converter implementation is thread-safe.

Several converter implementations are provided in the core.convert.support package as
a convenience. These include converters from Strings to Numbers and other common types.
Consider StringToInteger as an example for a typical Converter implementation:

9.5.3 GenericConverter

When you require a sophisticated Converter implementation, consider the GenericConverter
interface. With a more flexible but less strongly typed signature, a GenericConverter
supports converting between multiple source and target types. In addition, a
GenericConverter makes available source and target field context you can use when
implementing your conversion logic. Such context allows a type conversion to be driven
by a field annotation, or generic information declared on a field signature.

To implement a GenericConverter, have getConvertibleTypes() return the supported
source→target type pairs. Then implement convert(Object, TypeDescriptor,
TypeDescriptor) to implement your conversion logic. The source TypeDescriptor provides
access to the source field holding the value being converted. The target TypeDescriptor
provides access to the target field where the converted value will be set.

A good example of a GenericConverter is a converter that converts between a Java Array
and a Collection. Such an ArrayToCollectionConverter introspects the field that declares
the target Collection type to resolve the Collection’s element type. This allows each
element in the source array to be converted to the Collection element type before the
Collection is set on the target field.

Note

Because GenericConverter is a more complex SPI interface, only use it when you need it.
Favor Converter or ConverterFactory for basic type conversion needs.

ConditionalGenericConverter

Sometimes you only want a Converter to execute if a specific condition holds true. For
example, you might only want to execute a Converter if a specific annotation is present
on the target field. Or you might only want to execute a Converter if a specific method,
such as a static valueOf method, is defined on the target class.
ConditionalGenericConverter is the union of the GenericConverter and
ConditionalConverter interfaces that allows you to define such custom matching criteria:

A good example of a ConditionalGenericConverter is an EntityConverter that converts
between an persistent entity identifier and an entity reference. Such a EntityConverter
might only match if the target entity type declares a static finder method e.g.
findAccount(Long). You would perform such a finder method check in the implementation of
matches(TypeDescriptor, TypeDescriptor).

9.5.4 ConversionService API

The ConversionService defines a unified API for executing type conversion logic at
runtime. Converters are often executed behind this facade interface:

Most ConversionService implementations also implement ConverterRegistry, which
provides an SPI for registering converters. Internally, a ConversionService
implementation delegates to its registered converters to carry out type conversion logic.

A robust ConversionService implementation is provided in the core.convert.support
package. GenericConversionService is the general-purpose implementation suitable for
use in most environments. ConversionServiceFactory provides a convenient factory for
creating common ConversionService configurations.

9.5.5 Configuring a ConversionService

A ConversionService is a stateless object designed to be instantiated at application
startup, then shared between multiple threads. In a Spring application, you typically
configure a ConversionService instance per Spring container (or ApplicationContext).
That ConversionService will be picked up by Spring and then used whenever a type
conversion needs to be performed by the framework. You may also inject this
ConversionService into any of your beans and invoke it directly.

Note

If no ConversionService is registered with Spring, the original PropertyEditor-based
system is used.

To register a default ConversionService with Spring, add the following bean definition
with id conversionService:

A default ConversionService can convert between strings, numbers, enums, collections,
maps, and other common types. To supplement or override the default converters with your
own custom converter(s), set the converters property. Property values may implement
either of the Converter, ConverterFactory, or GenericConverter interfaces.

For most use cases, the convert method specifying the targetType can be used but it
will not work with more complex types such as a collection of a parameterized element.
If you want to convert a List of Integer to a List of String programmatically,
for instance, you need to provide a formal definition of the source and target types.

Fortunately, TypeDescriptor provides various options to make that straightforward:

Note that DefaultConversionService registers converters automatically which are
appropriate for most environments. This includes collection converters, scalar
converters, and also basic Object to String converters. The same converters can
be registered with any ConverterRegistry using the staticaddDefaultConverters
method on the DefaultConversionService class.

Converters for value types will be reused for arrays and collections, so there is
no need to create a specific converter to convert from a Collection of S to a
Collection of T, assuming that standard collection handling is appropriate.

9.6 Spring Field Formatting

As discussed in the previous section, core.convert is a
general-purpose type conversion system. It provides a unified ConversionService API as
well as a strongly-typed Converter SPI for implementing conversion logic from one type
to another. A Spring Container uses this system to bind bean property values. In
addition, both the Spring Expression Language (SpEL) and DataBinder use this system to
bind field values. For example, when SpEL needs to coerce a Short to a Long to
complete an expression.setValue(Object bean, Object value) attempt, the core.convert
system performs the coercion.

Now consider the type conversion requirements of a typical client environment such as a
web or desktop application. In such environments, you typically convert from String
to support the client postback process, as well as back to String to support the
view rendering process. In addition, you often need to localize String values. The more
general core.convert Converter SPI does not address such formatting requirements
directly. To directly address them, Spring 3 introduces a convenient Formatter SPI that
provides a simple and robust alternative to PropertyEditors for client environments.

In general, use the Converter SPI when you need to implement general-purpose type
conversion logic; for example, for converting between a java.util.Date and and
java.lang.Long. Use the Formatter SPI when you’re working in a client environment, such
as a web application, and need to parse and print localized field values. The
ConversionService provides a unified type conversion API for both SPIs.

9.6.1 Formatter SPI

The Formatter SPI to implement field formatting logic is simple and strongly typed:

To create your own Formatter, simply implement the Formatter interface above.
Parameterize T to be the type of object you wish to format, for example,
java.util.Date. Implement the print() operation to print an instance of T for
display in the client locale. Implement the parse() operation to parse an instance of
T from the formatted representation returned from the client locale. Your Formatter
should throw a ParseException or IllegalArgumentException if a parse attempt fails. Take
care to ensure your Formatter implementation is thread-safe.

Several Formatter implementations are provided in format subpackages as a convenience.
The number package provides a NumberFormatter, CurrencyFormatter, and
PercentFormatter to format java.lang.Number objects using a java.text.NumberFormat.
The datetime package provides a DateFormatter to format java.util.Date objects with
a java.text.DateFormat. The datetime.joda package provides comprehensive datetime
formatting support based on the Joda Time library.

Parameterize A to be the field annotationType you wish to associate formatting logic
with, for example org.springframework.format.annotation.DateTimeFormat. Have
getFieldTypes() return the types of fields the annotation may be used on. Have
getPrinter() return a Printer to print the value of an annotated field. Have
getParser() return a Parser to parse a clientValue for an annotated field.

The example AnnotationFormatterFactory implementation below binds the @NumberFormat
Annotation to a formatter. This annotation allows either a number style or pattern to be
specified:

Format Annotation API

A portable format annotation API exists in the org.springframework.format.annotation
package. Use @NumberFormat to format java.lang.Number fields. Use @DateTimeFormat to
format java.util.Date, java.util.Calendar, java.util.Long, or Joda Time fields.

The example below uses @DateTimeFormat to format a java.util.Date as a ISO Date
(yyyy-MM-dd):

9.6.3 FormatterRegistry SPI

The FormatterRegistry is an SPI for registering formatters and converters.
FormattingConversionService is an implementation of FormatterRegistry suitable for
most environments. This implementation may be configured programmatically or
declaratively as a Spring bean using FormattingConversionServiceFactoryBean. Because
this implementation also implements ConversionService, it can be directly configured
for use with Spring’s DataBinder and the Spring Expression Language (SpEL).

As shown above, Formatters can be registered by fieldType or annotation.

The FormatterRegistry SPI allows you to configure Formatting rules centrally, instead of
duplicating such configuration across your Controllers. For example, you might want to
enforce that all Date fields are formatted a certain way, or fields with a specific
annotation are formatted in a certain way. With a shared FormatterRegistry, you define
these rules once and they are applied whenever formatting is needed.

9.6.4 FormatterRegistrar SPI

The FormatterRegistrar is an SPI for registering formatters and converters through the
FormatterRegistry:

A FormatterRegistrar is useful when registering multiple related converters and
formatters for a given formatting category, such as Date formatting. It can also be
useful where declarative registration is insufficient. For example when a formatter
needs to be indexed under a specific field type different from its own <T> or when
registering a Printer/Parser pair. The next section provides more information on
converter and formatter registration.

9.6.5 Configuring Formatting in Spring MVC

9.7 Configuring a global date & time format

By default, date and time fields that are not annotated with @DateTimeFormat are
converted from strings using the DateFormat.SHORT style. If you prefer, you can
change this by defining your own global format.

You will need to ensure that Spring does not register default formatters, and instead
you should register all formatters manually. Use the
org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar or
org.springframework.format.datetime.DateFormatterRegistrar class depending on whether
you use the Joda Time library.

For example, the following Java configuration will register a global ' `yyyyMMdd’
format. This example does not depend on the Joda Time library:

Joda Time provides separate distinct types to represent date, time and date-time
values. The dateFormatter, timeFormatter and dateTimeFormatter properties of the
JodaTimeFormatterRegistrar should be used to configure the different formats for each
type. The DateTimeFormatterFactoryBean provides a convenient way to create formatters.

If you are using Spring MVC remember to explicitly configure the conversion service that
is used. For Java based @Configuration this means extending the
WebMvcConfigurationSupport class and overriding the mvcConversionService() method.
For XML you should use the 'conversion-service' attribute of the
mvc:annotation-driven element. See Section 22.16.3, “Conversion and Formatting” for details.

9.8 Spring Validation

Spring 3 introduces several enhancements to its validation support. First, the JSR-303
Bean Validation API is now fully supported. Second, when used programmatically, Spring’s
DataBinder can now validate objects as well as bind to them. Third, Spring MVC now has
support for declaratively validating @Controller inputs.

9.8.1 Overview of the JSR-303 Bean Validation API

JSR-303 standardizes validation constraint declaration and metadata for the Java
platform. Using this API, you annotate domain model properties with declarative
validation constraints and the runtime enforces them. There are a number of built-in
constraints you can take advantage of. You may also define your own custom constraints.

To illustrate, consider a simple PersonForm model with two properties:

publicclass PersonForm {
private String name;
privateint age;
}

JSR-303 allows you to define declarative validation constraints against such properties:

When an instance of this class is validated by a JSR-303 Validator, these constraints
will be enforced.

For general information on JSR-303/JSR-349, see the Bean
Validation website. For information on the specific capabilities of the default
reference implementation, see the Hibernate
Validator documentation. To learn how to setup a Bean Validation p